Recent Development in Optical Fiber Biosensors

Remarkable developments can be seen in the field of optical fibre biosensors in the last decade. More sensors for specific analytes have been reported, novel sensing chemistries or transduction principles have been introduced, and applications in various analytical fields have been realised. This review consists of papers mainly reported in the last decade and presents about applications of optical fiber biosensors. Discussions on the trends in optical fiber biosensor applications in real samples are enumerated.


Introduction
Biosensor development is driven by the continuous need for simple, rapid, and continuous in-situ monitoring techniques in a broad range of areas, e.g. medical, pharmaceutical, environmental, defense, bioprocessing, or food technology.
Biosensors make use of biological components in order to sense a species of interest (which by itself need not be a "biospecies"). On the other side, chemical sensors not using a biological component but placed in a biological matrix are not biosensors by definition. Biological systems (such as tissues, micro-organisms, enzymes, antibodies, nucleic acids, etc.) when combined with a physico-chemical transducer (optical, electrochemical, thermometric, piezoelectric) form a biosensor.
On the other hand, the development of optical-fiber sensors during recent years is related to two of the most important scientific advances: the laser and modern low-cost optical fibers. Recently, optical fibers have become an important part of sensor technology. Their use as a probe or as a sensing element is increasing in clinical, pharmaceutical, industrial and military applications. Excellent light delivery, long interaction length, low cost and ability not only to excite the target molecules but also to capture the emitted light from the targets are the main points in favour of the use of optical fibers in biosensors.
Optical fibers transmit light on the basis of the principle of total internal reflection (TIR). Fiber optic biosensors are analytical devices in which a fiber optic device serves as a transduction element. The usual aim is to produce a signal that is proportional to the concentration of a chemical o biochemical to which the biological element reacts. Fiber optic biosensors are based on the transmission of light along silica glass fiber, or plastic optical fiber to the site of analysis. Optical fiber biosensors can be used in combination with different types of spectroscopic technique, e.g. absorption, fluorescence, phosphorescence, surface plasmon resonance (SPR), etc.
Optical biosensors based on the use of fiber optics can be classified into two different categories: intrinsic sensors, where interaction with the analyte occurs within an element of the optical fiber; and extrinsic sensors, in which the optical fiber is used to couple light, usually to and from the region where the light beam is influenced by the measurand.
Biosensors are attractive because they can be easily used by non-specialist personnel and they allow accurate determination with either no or minimal sample treatment. Therefore, fiber optic biosensors may be especially useful in routine tests, patient home care, surgery and intensive care, as well as emergency situations.

Absorbance measurements
The simplest optical biosensors use absorbance measurements to determine any changes in the concentration of analytes that absorb a given wavelength of light. The system works by transmitting light through an optical fiber to the sample; the amount of light absorbed by the analyte is detected through the same fiber or a second fiber. The biological material is immobilized at the distal end of the optical fibers and either produces or extracts the analyte that absorbs the light.
A fiber optic pH sensor [1] and a fiber optic oxygen sensor [2] have been developed by Wolthuis et al., for use in medical applications. In the first case, the sensor uses an absorptive indicator compound with a long wavelength absorption peak near 625 nm; change in absorption over the pH range 6.8 to 7.8 is reasonably linear. The sensor is interrogated by a pulsed, red LED. Return light signal is split into short and long wavelength components with a dichroic mirror; the respective signals are detected by photodiodes, and their photocurrents are used to form a ratiometric output signal. In laboratory tests, the sensor system provided resolution of 0.01 pH and response time of 30-40 s. Following gamma sterilization, laboratory sensor testing with heparinised human blood yielded excellent agreement with a clinical blood gas analyzer. Excellent sensor performance and low cost, solid-state instrumentation are hallmarks of this sensor-system design. In the second, the sensor's viologen indicator becomes strongly absorbant after brief UV stimulation, and then returns to the transparent state. The rate of indicator return to transparency is proportional to the local oxygen concentration. Indicator absorbance is monitored with a red LED. The solid state sensor system has performance comparable to existing oxygen measurement techniques, and may be applicable for both in vitro and in vivo oxygen measurements.
Molecular recognition processes are very important in the investigation of biological phenomena as well as development of fiber optic biosensor. As promising materials, biofunctionalized polydiacetylene lipids have many special properties. p-10,12-pentacosadiyne-1-N-(3,6,9trioxaundecylanide) -α-D-mannopyranoside (MPDA) was synthesized from 10,12-pentacasadiynoic acid (PDA). The mixed monolayer MPDA/PDA was prepared by using Langmuir-Blodgett technique on glass and optical fiber, respectively. Molecular recognition of Escherichia coli (E. coli) resulted in a change of the film colour and can be quantified by UV-VIS absorption spectroscopy. The properties might be used for screening drugs and diagnosis [3].
On the other hand, thin films of polyaniline (PANI) and polypyrrole (PPy) have been shown to respond to pH and to the redox state of a solution, thereby undergoing spectral changes in the visible and near infrared. They also may serve as a matrix for enzyme immobilization. PANI films are easily prepared, and spectral changes depending on analyte concentration occur at wavelengths at which low cost lasers and LEDs are available. PANI and PPy are conductive organic polymers that can be driven between different oxidation states by chemical or electrochemical methods. PANI has two redox equilibriums associated with the polymer chain, and the optical properties of the polymer are functions of the state of protonation and the oxidation state. Since protons and electrons are directly involved in the polymer redox reaction, changes in optical spectra (e.g. absorbance) can be attributed to the concentration of protons or the number of electrons produced in an enzymatic reaction. PANI films of good optical quality can be produced by chemical means on almost any desired support including polystyrene, polycarbonate or glass. However, both optical and electrochemical PANI sensors tend to drift (in terms of conductivity or absorbance) in the order of 1% of the background signal per day. This makes daily recalibration necessary prior to measurements. Microtiterplates (micro-well) assays offer an alternative to conventional sensing because calibrators can be placed in one or more wells while actual assays are performed in the residual wells. This is one of the reasons why micro-well assays are commonly employed in routine analysis. In this way, a technique for coating the wells of microtiterplates with polyaniline layers and with polyaniline/enzyme layers is presented by Piletsky et al. [4]. The resulting wells are shown to be useful for assaying enzyme substrates (as exemplified for glucose via pH) and hydrogen peroxide (via the redox properties of the film). Analyte detection is based on monitoring the absorption spectra of the polyaniline, which turn purple as a result of redox processes, or green on formation of acids, by enzymatic reactions. Hydrogen peroxide (a species produced by all oxidases) and glucose (which yields protons on enzymatic oxidation) have been determined in the millimolar to micromolar concentration range.
Recently, Llobera et al. [5] present the characterization and optimization of flexible transducers with demultiplexing properties, suitable for on-chip detection. The micro-system consists of a hollow prism that can be filled with the fluid to be investigated. Two 2D (cylindrical) biconvex lenses modify the optical path before and after propagating through the prism, having parallel beams inside the prism and focusing on the output optical fiber. Light is coupled into the system through a multimode optical fiber inserted into a channel. This channel enables the exact positioning of the fiber with respect to the biconvex lens, conferring the device with a simple, yet effective, self-alignment system. The same applies for the output fiber which enables the collected light to reach the photo-detector. The optimization of the hollow prisms has been done by measuring the absorbance as a function of the concentration for fluorescein and methylorange. Methylorange absorbs blue-green (λ = 490-510 nm) or blue (λ = 435-480 nm) light depending if its molecules are in its acidic or basic form, respectively. When a fixed concentration of methylorange is mixed with buffer with different pH, the fraction of each form of methylorange varies, causing a variation of the absorption spectrum of the solution. The working wavelength used in this work is only absorbed by the basic form (λ = 460 nm), hence, its absorbance will be minimum for low pH values. As the fraction of methylorange in its basic form increases (that is, as the pH increases), the absorbance of the working wavelength will increase. Hence, pH can be measured as a function of the absorbance of the methylorange in its basic form. Results show how the limit of detection (LOD) for fluorescein and methylorange diluted in phosphate buffer can be significantly lowered, by increasing the size of the prism or increasing the total deviation angle (the measurements showed a LOD in the µM range for both species).
Surface plasmon resonance (SPR) is an optical phenomenon caused by charge density oscillation at the interface of two media with dielectric constants of opposite sign, for example a metal and a dielectric. In this way, by tuning the plasmon resonance to a wavelength for which the outer medium is absorptive, a significant variation of the spectral transmittance of the device is produced as a function of the concentration of the analyte. With this mechanism, selectivity can be achieved without the need of any functionalization of the surfaces or the use of recognizing elements, which is a very interesting feature for any kind of chemical sensor or biosensor. Doubly deposited uniform-waist tapered fibers are well suited for the development of these new sensors. Multiple surface plasmon resonance, obtainable in those structures, can be used for the development of microspectrometers based on this principle [6].
Since the coming of the FTIR spectrometers, infrared spectroscopy has become an indispensable and efficient tool of analytical studies. Nevertheless, classical attenuated total reflection (ATR) or transmission registration need also to collect samples. An alternative method to acquire the infrared spectra consists of using some optical fiber, avoiding then the samplings. For this, the fiber is employed, on the one hand, to transmit the IR beams from the spectrometer to the sample, and on the other hand, as a probe by inserting a part of the fiber, called the sensing zone, into the studied environment. This technique is called fiber evanescent wave spectroscopy (FEWS) for fiber evanescent wave spectroscopy because it is generally considered that the principle of the measurement is based on the presence of evanescent wave around the fiber during the propagation of light into the fiber. A new generation of optical fibers has been developed based on the large transparency domain of an original family of IR chalcogenide glasses transmitting from 2 to about 12 µm.
Fiber-based infrared sensing has been established as an efficient, non-destructive and selective technique for the detection of organic and biological species. This technique combines the benefits of ATR spectroscopy with the flexibility of using a fiber as the transmission line of the optical signal, which allows for remote analysis during field measurements or in clinical environments. The sensing mechanism is based on the absorption of the evanescent electric field, which propagates outside the surface of the fiber and interacts with any absorbing species at the fiber interface. This mechanism is analogous to that observed with an ATR crystal; however the fiber geometry creates a large number of internal reflections which enhances detection sensitivity. The availability of fibers with high infrared transmission in the spectral region between 400 and 4000 cm −1 allows one to collect the highly specific vibrational spectrum of organic chemicals and biomolecules. This technique, known as fiber evanescent wave spectroscopy (FEWS), has been applied to the detection of a wide range of chemicals and pollutants.
Since this spectral range comprises the "fingerprint" region of biomolecules, the FEWS technique has also shown promise for biomedical and clinical applications. It can be used as an efficient tool for chemical analysis of bio-fluids [7]. More recently, IR fiber sensors have been applied to monitor the metabolism of live whole cells. This ability to monitor metabolic processes in live cells has interesting potential for the design of bio-optic sensors. Disruption of the cell metabolism can be observed in response to minute amounts of toxicants, which would otherwise be far below the detection limit of IR spectroscopy. In effect, the cells act as a sensitizer for the IR sensor. Additionally, the process of monitoring a cell response permits detection of a wide range of compounds, which may have similar toxicological activities but different molecular structure. Hence biochemicals are detected based on their activity rather than identity, an important distinction critical in the design of sensors which may be challenged with a wide range of analytes to be detected.
Chalcogenide glasses are an ideal choice for the design of IR fiber bio-sensors based on materials properties. Mid-infrared fibers have been used previously for the characterization of live biological samples. An IR fiber optic neurotoxin biosensor was constructed by applying a biologically active cladding to the core of an infrared transmitting chalcogenide fiber [8]. Binding of the surface bound receptor protein was monitored by performing infrared difference spectroscopy on the fiber optic probe before and after its exposure to various concentrations of neurotoxin in solution. Signals measuring conformational changes as a result of these interactions are observed to saturate in agreement with established biochemical kinetics for the receptor. Fiber-optic components are shown to be much more sensitive than bulk optical components in performing these measurements. These fibers have also been functionalized with biological surface coatings such as enzyme films for selective bio-sensing [9]. In order to immobilize glucose oxidase on the surface of such an IR-transparent wave guide, crystalline bacterial cell surface layers (S-layers) were used as a carrier, instead of using silanes as an enzyme coupler as frequently described in the literature. S-layer proteins, which have the capability for selfassembling on suitable surfaces, were cross-linked and further activated with glutaraldehyde before the immobilization procedure. The reactive enzyme layer coating the core of the fiber serves to catalyze chemical reactions specifically when the fiber is used as chemical sensor. The chalcogenide fiber was coupled to a Fourier transform infrared (FT-IR) spectrometer which yielded spectra at various stages of the chemical processes as well as developments of signal bands as a function of time. The fiber was used as ATR element and could provide evanescent-field IR spectra in the range of 4000-800 cm -1 of the covering surface film thickness estimated at ∼ 40 nm. All experimental surface modifications were carried out in situ in a 12-cm-long flow cell into which the fiber was positioned initially.
Lucas et al. [10] functionalize the surface of chalcogenide Te-As-Se fibers with live human lung cells that can act as sensitizer for the detection on micromolar quantities of toxic agents. First it presents how the hydrophobic behavior of chalcogenide glass affects the spectroscopic properties of chalcogenide fibers. Then it presents initial results on the variation of cell spectra in response to various toxic agents. This study emphasizes the potentials of chalcogenide fibers for the design of cellbased bio-optic sensors.
Near-infrared spectroscopy (NIRS) use depends on the relatively good transparency of biological tissue in the near-infrared range, which allows for transmission of photons through the tissue, so that they can be detected at the exit from the tissue. In particular, oxygenated haemoglobin (HbO2) and deoxy-haemoglobin (Hb) are the dominant absorbing elements between 700 and 1000 nm, and the transmission of light is relatively unaffected by water in the same region. Thus, the near-infrared region of the spectrum is the most favourable to the optical measurement of these parameters, so that NIRS provides a non-invasive, non-ionizing means to monitor total haemoglobin concentration (HbO2 + Hb) that is considered as total blood volume (HbT or V) as well as oxygen saturation in the living tissue. Optic fiber probes were used as the optical head of a novel, highly sensitive near-infrared continuous wave spectroscopy (CW-NIR) instrument. This prototype was designed for non-invasive analysis of the two main forms of haemoglobin [11].

Reflectance measurements
Interest has been shown in the detection of free radicals in view of the evidence implicating a primary or secondary role for them in the initiation or progression of many diseases, the majority of which are characterized by an inflammatory reaction. Naughton et al. [12] describe the development of a fibre optic sensor, based on immobilized nitrophenol that is of potential use for the continuous monitoring of OH radical production. This reflectance based sensor incorporates an OH radical sensitive chromophore, affording a decrease in its reflectance spectrum upon attack by this extremely reactive oxygen-derived radical attributable to the formation of nitrocatechol. Nitrophenol was immobilized onto XAD-7 methacrylate beads. Subsequently the beads were attached to the distal end of a polymethylmethacrylate fiber optic. Nitrocatechol, generated from the attack of OH radical on nitrophenol, exhibits a strong absorption band in the visible region of the electromagnetic spectrum (λ (max) = 510 nm).
Dyr et al. [13,14] evaluated the feasibility to follow the enzymatic conversion of surface-bound fibrinogen by thrombin to fibrin monomer and possible complex formation between surface-bound fibrin monomer and fibrinogen in solution using a SPR sensor. For the investigations of optical properties of immobilised molecular layers, the Kretschmann geometry of the attenuated total reflection (ATR) method was utilised. Density of fibrinogen bound to the surface depended on the concentration of fibrinogen in solution during the adsorption process. A fibrinogen monolayer was always formed. Fibrinogen in solution did not bind to surface-bound fibrinogen. Bound fibrinogen converted by thrombin to fibrin monomer interacted (rather slowly) with fibrinogen in solution. The rate of adsorption depended upon immobilised fibrin monomer density, fibrinogen concentration in solution, and on the presence of calcium ions. At low fibrin monomer density, the second layer was formed that contained about the same amount of protein as the first layer, at higher fibrin monomer concentration less than one molecule of fibrinogen per molecule of fibrin monomer was captured.
A novel approach for the detection of molecular interactions in which a colorimetric resonant diffractive grating surface is used as a surface binding platform is proposed by Cunningham et al. [15]. The grating, when illuminated with white light, is designed to reflect only a single wavelength. When molecules are attached to the surface, the reflected wavelength is shifted due to the change of the optical path of light that is coupled into the grating. By linking receptor molecules to the grating surface, complementary binding molecules can be detected without the use of any kind of fluorescent probe or particle label. The detection technique is capable of resolving changes of ∼ 0.1 nm thickness of protein binding, and can be performed with the grating surface either immersed in fluid or dried. The readout system consists of a white light lamp that illuminates a small spot of the grating at nominally normal incidence through a fiber optic probe, and a spectrometer that collects the reflected light through a second fiber, also at normal incidence. Because no physical contact occurs between the excitation/readout system and the grating surface, no special coupling prisms are required and the grating can be easily adapted to any commonly used assay platform, such as microtiter plates and microarray slides. A single spectrometer reading may be performed in several milliseconds, thus it is possible to quickly measure a large number of molecular interactions taking place in parallel upon a grating surface, and to monitor reaction kinetics in real time.
Based on the principle of multiple-reflection in white-light interferometry, an optical fiber immobilized with protein molecule (antigen or antibody) is adopted as a high sensitivity biosensor to accomplish the real time monitor of the immunoassay [16][17][18]. The experimental results of rabbit-IgG with anti-rabbit-IgG immunoreaction show that by the aid of this biosensor, a biolayer change of approximately 0.70 nm in optical thickness can be detected by measuring the reflected spectrum phase shifting between the two interfaces of biolayer. Compared with others, this method has advantages, such as simple structure, lower cost, high sensitivity and reliability, label-free in immunoassay and direct monitor the immunoassay. This biosensor is easy to be integrated as a BIAcore (biomolecular interaction analysis core) for parallel direct detection of antigen-antibody interactions. SPR biosensor has been widely used in the last 10 years to analyze the affinity between ligand and analyte [19]. The SPR condition is usually achieved in the attenuated total reflection (ATR) geometry by using prism-coupling optics. This optical setup is usually bulky and is incompatible with micrometer-sized probes. In place of the conventional SPR, interest in localized plasmon resonance (LPR) has grown in recent years, since LPR is quite compatible with such microsensors. The LPR of gold appears as an absorption band at about 550 nm in the presence of thin molecular layers on gold. In this condition, the LPR produces a large enhancement of the electric field around small gold particles or roughness of a gold surface, so that it is applicable in a highly sensitive affinity sensor of size less than microns. Kajikawa et al. propose a simple and sensitive optical sensing method for biological applications [20,21]. Since gold behaves as a dielectric with a large extinction index under blue or violet light, presence of a transparent surface layer on gold produces a large decrease in the reflectivity of the gold surface due to multiple reflections in the surface layer. Their call this phenomenon anomalous reflection (AR) of the gold surface. AR is applicable to affinity biosensors based on fiber optics, so that inexpensive and disposable micrometer-sized biosensor probes are possible based on this technique. Here, the authors demonstrate the application of AR to real-time measurements of the adsorption process of octadecanethiol on gold and the affinity of streptavidin to a biotin-labeled monomolecular layer on gold.
Misiakos et al. [22] describe an optical affinity sensor based on a monolithic optoelectronic transducer, which integrates on a silicon die thin optical fibers (silicon nitride) along with self-aligned light-emitting diodes (LED) and photodetectors (silicon p/n junction). The LEDs are optically coupled to the corresponding photodetectors through silicon nitride fibers. Specially designed spacers provide for the smooth bending of the fiber at its end points and toward the light source and the detector ensuring high coupling efficiency. The transducer surface is hydrophilized by oxygen plasma treatment, silanized with (3-aminopropyl)triethoxysilane and bioactivated through adsorption of the biomolecular probes. The use of a microfluidic module allows real-time monitoring of the binding reaction of the gold nanoparticle-labeled analytes with the immobilized probes. Their binding within the evanescent field at the surface of the optical fiber causes attenuated total reflection of the waveguided modes and reduction of the detector photocurrent. The biotin-streptavidin model assay was used for the evaluation of the analytical potentials of the device developed. Detection limits of 3.8 and 13 pM in terms of gold nanoparticle-labeled streptavidin were achieved for continuous and stoppedflow assay modes, respectively. The detection sensitivity was improved by silver plating of the immobilized gold nanoparticles, and a detection limit of 20 fM was obtained after 20-min of silver plating. In addition, two different analytes, streptavidin and antimouse IgG, were simultaneously assayed on the same chip demonstrating the multianalyte potential of the sensor developed.
Recently, an optical fiber biosensor is introduced herein, which could directly detect biological interaction such as immunoreactions of antigens and antibodies without destroy the biolayer [23]. The test is based on the theory of multilayer-reflection principle in white-light interferometry. When immunoreactions occur, the reflected spectrum phase shifts. Immunoreactions could be detected by means of reflected spectrum phase shifting, or by biolayer thickness changing. Continuously detecting of thickness changing on a fractional nanometer scale with subsecond repetition times is allowed in this system. The detecting system has high sensitivity, high precision, and high speed, cost effective and working on a high reliability. The bioprobe is easy integrated as a BIAcore. The system and the experimental results on the reaction of rabbit-IgG with anti-rabbit-IgG are described in this work.

Fluorescence measurements
Fluorescence techniques provide sensitive detection of biomolecules. Furthermore, since fluorescence intensity is proportional to the excitation intensity, even weak signals can be observed. In last decade reagentless fiber-based biosensors have been developed. These biosensors are capable of detecting changes in cell behaviour, metabolism and cell death when exposed to toxic agents.
Fluorescence measurements are not used as often as absorbance and reflectance with enzyme optical fiber-based biosensors, as it is not common for enzyme reactions to produce fluorescent products or intermediates. Most fluorescence techniques employ a fluorescent dye to indirectly monitor formation or consumption of a transducer.
Several enzyme-catalysed reactions involve the production or consumption of fluorescent species. NAD + (nicotinamide adenine dinucleotide)-dependent reactions, catalysed by a group of dehydrogenase enzymes, are some of the notable examples of this chemical phenomenon. In the general dehydrogenase reaction: Substrate + NAD(P) + dehydrogenase product + NAD(P)H NADH or NADPH is produced, which can be detected using fibre optics through its fluorescence at λ ex = 350 nm, λ em = 450 nm. Several optical sensors have thus been produced by integrating different dehydrogenase or oxidoreductase enzymes with optical fibres. These sensors are listed in Table 1.
A fiber optic-based biosensor which integrates a novel array of optical and electrical components, together with long fused silica fibers and proteins for detection of analyte in solution has been proposed by Anderson et al. [98]. The optical fiber core near the distal end is tapered and coated with either antibodies or DNA binding proteins. Assays are performed by flow of a solution containing the fluorescently tagged ligand molecules over the coated fiber. Within seconds, analyte recognition occurs and a fluorescence signal is transmitted back up the fiber. Applications for the biosensor include clinical diagnostics, pollution control, and environmental monitoring.
The use of DNA as a selective recognition element in biosensor design is a new and exciting area in analytical chemistry. Experiments have been completed wherein ssDNA was covalently immobilized onto quartz optical fibers to successfully demonstrate the basis for development of an optical biosensor for DNA. The non-optimized configuration described was able to detect femtomolar concentrations of cDNA with an analysis time of less than 1 h. These experiments indicate that biosensors which can selectively detect genetic material from biological may now be created, with advantages of low detection limits, reasonable analysis times, highly stable biorecognition elements, and regenerability. An alternative approach which may be used to create biosensors with long term chemical stability takes advantage of the stability of DNA. With the recent advent of DNA probe technology, a number of selective oligomers which interact with the DNA of important biological species, for instance salmonella, have been identified. These have been used to provide a new type of selective biorecognition element which is highly selective, stable, and can be easily synthesized in the laboratory as compared to other chemically synthesized biorecognition elements. As a result, species specific DNA probes may be exploited for biosensor development.
Piunno et al. [99,100] reports one of the first biosensors for direct analysis of DNA hybridization by use of an optical fiber. ssDNA was covalently immobilized onto optical fibers by first activating the surface of the quartz optical fiber with a long chain aliphatic spacer arm terminated in a 5'-0-dimethoxytrityl-2'-deoxyribonucleoside, followed by automated solid-phase DNA synthesis. Detection of dsDNA at the fiber surface after hybridization between immobilized ssDNA and cDNA was achieved by exposing the complex to an ethidium bromide solution followed by washings with hybridization buffer solution. The ethidium cation (3,8-diamino-6-phenyl-5-ethyl-phenanthridium) is a fluorescent compound which strongly associates with dsDNA by intercalation into the base stacking region and, in some cases, the major groove of the double helical structure. It is shown that the fluorescence response of the ethidium cation can be monitored in a total internal reflection configuration along an optical fiber to quantify the presence of dsDNA at the surface of the fiber, with the fluorescence intensity being directly proportional to the amount of cDNA initially present in solution. This approach may be refined and used for rapid identification and quantitation of the presence of microorganisms such as pathogenic bacteria and viruses in bodily fluids, food and feed commodities, and may also find application in screening for genetic disorders.
Other fiber optic DNA biosensors and also other fiber optic biosensors based on fluorescence measurements are summarised in Table 1.
Optical biosensors based on fluorescence detection often use the combination of a fluorescent bioreceptor associated with an optical transducer. Fluorescent biosensors may also be obtained by immobilizing whole cells on the surface of a sensor layer. This bioactive layer is usually placed in front of the tip of an optical fibers bundle to generate a fluorescence signal. The optical fibers are required to send the excitation radiation to the fluorescent bioelement and convey the fluorescence radiation up to a fluorimeter. When algal cells are deposited on the surface of an opaque support, they are placed at some distance to the optical fibers bundle to allow the fluorescence radiation emitted by the illuminated area to be collected properly. In order to improve the simplicity and reliability of fluorescence based biosensors, optically translucent supports are used because their optical properties enable detection of fluorescence emitted by the algal cells even if they are entrapped in the bulk of the translucent support. Silica matrixes have interesting properties including optical translucence, biocompatibility and chemical inertness. The design flexibility of sol-gel technique and ease of fabrication can fulfil to create the support with structural and chemical features that could be compatible with biomaterials. While immobilization of enzymes and whole cells in sol-gel is a wellknown technique in biosensor application, it is not yet used for the construction of vegetal-cell optical biosensors. On the other hand, when an enzyme is immobilized, a substrate is required and a fluorescent indicator may be added to enable optical detection of the analyte. In the case of detection based on chlorophyll fluorescence, those reagents are not necessary, and a reagentless biosensor can be constructed for toxic chemicals determination. Recently, Nguyen-Ngoc and Tran-Minh investigated translucent matrixes obtained from a sol-gel process for entrapment of fluorescent biomaterials in order to improve the fluorescent biosensor fabrication. Vegetal cells as bioreceptors have been tested for herbicides determination [101].
Also, a single, compact probe containing a manifold of chemical and biological recognition principles that could all be simultaneously and independently interrogated forms an excellent foundation for the development of clinical and environmental total analysis systems. Imaging fibers consisting of a fused fiber bundle that can each be optically addressed independently are very useful for optical imaging applications. In this sense, optical imaging fibers with micrometer-sized wells were used as a sensing platform for the development of microarray optical ion sensors based on selective bulk extraction principles established earlier for optodes. Uniform 10 µm sized microspheres based on plasticized poly(vinyl chloride) containing various combinations of ionophores, fluoroionophores and lipophilic ion-exchangers were prepared for the detection of sodium, potassium, calcium and chloride, and deposited onto the wells of etched fiber bundles. The fluorescence emission characteristics of individual microspheres were observed from the backside of the fibers and were found to selectively and rapidly change as a function of the sample composition [102].

Chemiluminescence measurements
A molecular assembly in which a conjugated polymer is interfaced with a photodynamic protein is described by Ayyagari et al. [103]. The conjugated polymer, functionalized with biotin, is designed such that it can be physisorbed on or chemically grown off a glass surface. The streptavidin-derivatized protein is immobilized on the biotinylated polymer matrix through the strong biotin-streptavidin interactions. The assembly, built on the surface of an optical fiber or on the inside walls of a glass capillary form an integral part of a biosensor for the detection of environmental pollutants such as organophosphorus-based insecticides. The protein in the system can be replaced by any biological macromolecule of interest. These authors study one specific case, the inhibition of the enzyme alkaline phosphatase. The enzyme catalyzes a reaction producing an intermediate compound that chemiluminesces, and the chemiluminescence signal is monitored to detect and quantify insecticides such as paraoxon and methyl parathion. Preliminary results indicate ppb level detection with response time less than 1 minute.
Chen et al. [104][105][106] report a technique to immobilize a multilayer enzyme assembly on an optical fiber surface. A multilayer of an enzyme, alkaline phosphatase, was immobilized by chemical crosslinking on an optical fiber surface. Chemiluminescence, ellipsometry, and surface plasmon resonance were used to characterize the structure and activity of the assembly. A chemiluminescence-based fiber optic biosensor utilizing this immobilization technique has been developed for the detection of organophosphorus-based pesticides.
A fiber optic biosensor based on the electrochemiluminescence of luminol has been developed for glucose and lactate flow injection analysis by Marquette and Blum [107,108]. The electrochemiluminescence of luminol was generated using a glassy carbon electrode polarised at +425 mV vs. a platinum pseudo-reference electrode. After optimisation of the reaction conditions and physicochemical parameters influencing the sensor response, the measurement of hydrogen peroxide could be performed in the range 1.5 pmol-30 nmol. Glucose oxidase or lactate oxidase, were immobilised on polyamide and collagen membranes. With collagen as the enzymatic support, the detection limits for glucose and lactate were 60 pmol and 30 pmol, respectively, whereas with the enzymatic polyamide membranes, the corresponding values were 150 pmol and 60 pmol. A chemiluminescence biosensing system for antioxidants was developed by Palaoran et al. [109] based on luminol and hematin co-immobilized on a cellulose membrane disc. The concentration of the antioxidant was quantified through the measurement of the inhibition of the chemiluminescence emitted when hydrogen peroxide was introduced into the reagent phase. The instrumentation employed in the measurement was a fabricated luminometer employing optical fibers and a UV-enhanced photodiode transducer. The minimum detectable concentration was 100 µM, and the response time was less than 60 seconds.
The enhanced chemiluminescence (CL) reaction of the luminol-H 2 O 2 -horseradish peroxidase (HRP) system with immobilized HRP using microencapsulation in a sol-gel matrix has been used to develop a biosensor for p-iodophenol, p-coumaric acid, 2-naphthol and hydrogen peroxide [110]. The detection limits obtained for p-iodophenol, p-coumaric acid and 2-naphthol were 0.83 µM, 15 and 48 nM, respectively. Direct enzyme immobilization onto the end of the optical fibre permits the construction of a remote enhanced CL biosensor. This remote biosensor has been applied to hydrogen peroxide assay (detection limit 52.2 µM).
In other study, an optical fiber bienzyme sensor based on the luminol chemiluminescent reaction was developed and demonstrated to be sensitive to glucose [111]. Glucose oxidase and horseradish peroxidase were co-immobilized by microencapsulation in a sol-gel film derived from tetraethyl orthosilicate. The calibration plots for glucose were established by the optical fiber glucose sensor fabricated by attaching the bienzyme silica gel onto the glass window of the fiber bundle. The linear range was 0.2-2 mM and the detection limit was approximately 0.12 mM.
Recently, Magrisso et al. [112] describe the construction of a novel computerized multi-sample temperature-controlled luminometer for a fiber array-based biosensor to monitor circulating phagocyte activity. It can perform simultaneously integral measurements of chemiluminescence emitted from up to six samples containing less that 0.5 µl whole blood while the samples and detector do not change their position during the measurement cycle. The optical fibers in this luminometer are used as both light guides and solid phase sample holders. The latter feature of the instrument design simplifies the assessment process of both the extra-cellular and the intra-cellular parts of the phagocyte-emitted chemiluminescence using the same system. This new technology may find use in a wide range of analytical luminescence applications in biology, biophysics, biochemistry, toxicology and clinical medicine.

Bioluminescence measurements
A biosensor associates a bioactive sensing layer with a suitable transducer giving a usable output signal. Although the selective molecular recognition of the target analyte can theoretically be achieved with various kinds of affinity systems, in most biosensors, enzymes are concerned. The biocatalysts are generally immobilized on an artificial support placed in close contact with the transducer. Covalent immobilization is preferable because it prevents the leakage of enzymes from the support. This can now be easily achieved with the availability of commercially preactivated membranes. Sensing layers prepared with such membranes were used for the development of amperometric as well as luminescence-based fiber-optic biosensors. In addition to the target analyte, enzymatic reactions generally involve one or two other substrates, which must be added to the reaction medium when an enzyme based biosensor is operated. The sensing scheme, and therefore the transducer associated with the sensing layer, determines the sensitivity of an enzyme-based sensor. Due to the peculiar nature of one of the reaction products, i.e., light, which can be detected at a very low level, bioluminescence and chemiluminescence reactions associated with an optical transduction can be used to design highly sensitive biosensors.
In 1990, Gautier et al. [113] investigated highly selective and ultra-sensitive biosensors based on luminescent enzyme systems linked to optical transducers. In this way, a fiber optic sensor with immobilized enzymes was designed; the solid-phase bio-reagent was maintained in close contact with the tip of a glass fibre bundle connected to the photomultiplier tube of a luminometer. A bacterial luminescence fiber optic sensor was used for the determination of NADH. Various NAD(P)-dependent enzymes, sorbitol dehydrogenase, alcohol dehydrogenase and malate dehydrogenase, were coimmobilized on preactivated polyamide membranes with the bacterial system and used for the determination of sorbitol, ethanol and oxaloacetate at the nanomolar level with a good precision. The same authors, also in 1990, described a multi-function biosensor for the determination of either ATP or NADH using a single bioluminescence-based fiber optic probe [114]. This was made possible by coimmobilizing the firefly luciferase from Photinus pyralis for ATP analysis with the bacterial luciferase/oxidoreductase system from Vibrio harveyi for NADH analysis, on the same pre-activated polyamide membrane. In 1992, Gautier describes the role of membranes in the design of the main types of biosensors proposed in this time [115].
The reproducible and easy immobilization of receptors on sensor surfaces is a prerequisite for the development of receptor-based fiber optic biosensors. Using a fused silica fiber as the transducer, binding processes of luminescently labeled ligands can be monitored by evanescent wave sensor technology. The vesicle fusion technique was chosen for the immobilization of membrane-bound receptors in order to preserve their binding specificity and activity, by embedding them in an environment similar to a lipid bilayer. The results of initial studies of repetitive cycles of lipid layer deposition and removal, indicating good reproducibility of lipid layer formation on the fiber, are presented by Klee et al. [116]. Using the binding of fluorescently labeled streptavidin to a biotinylated lipid layer as a model system for receptor-ligand interaction, good sensitivity, combined with low nonspecific binding were observed.
The characteristics and performance of biosensors mainly depend on the properties of the bioactive layer associated with the transducer. Two approaches are addressed by Blum et al. [117]: the designs of a) a reagentless fiber-optic biosensor with cosubstrates embedded in the vicinity of the immobilized enzyme; b) a compartmentalized layer with sequential enzymatic reactions for improved signal detection. Concerning the reagentless biosensor, Blum et al. focus on the bacterial bioluminescent system involving two enzymes, oxidoreductase and luciferase, for NADH detection. When associating such a self-contained sensing layer with the transducer, the biosensor can be operated for 1.5 h without reloading. For the studies of compartmentalized enzyme layers, the sequential bienzymatic system lactate oxidase-peroxidase is chosen as a model. The hydrogen peroxide produced by the lactate oxidase reaction serves as a cosubstrate for the chemiluminescence reaction of luminol catalyzed by peroxidase. Compartmentalization of the bioactive layer is obtained by immobilizing the two enzymes separately on different membranes stacked at the sensing tip of the fiber-optic sensor. When using such a design, a 20-fold increase of the sensor response for lactate is obtained compared with a sensor including lactate oxidase and peroxidase randomly coimmobilized on the same membrane. Latter, the same research group developed a fiber optic biosensor for the specific and alternate determination of ATP, ADP and AMP [118]. The sensing layer is arranged by compartmentalizing the tri-enzyme sequence adenylate kinase -creatine kinase -firefly luciferase. The two kinases are covalently coimmobilized on a collagen membrane, whereas firefly luciferase is bound alone on a separate one. For the specific determination of each adenylic nucleotide, three particular reaction media are needed with which flow-injection analysis can be performed in the 2.5-2500 pmol for ATP, 10-2500 pmol for ADP and 25-5000 pmol for AMP linear ranges. When the three nucleotides are present simultaneously in the same sample, the transient inhibition of adenylate-kinase activity by adenosine 5'-monosulphate enables their specific and alternate measurement.
Also, a portable biosensor has been developed to meet the demands of field toxicity analysis [119]. This biosensor consists of three parts, a freeze-dried biosensing strain within a vial, a small light-proof test chamber, and an optic-fiber connected between the sample chamber and a luminometer. Various genetically engineered bioluminescent bacteria were freeze-dried to measure different types of toxicity based upon their modes of action. GC2 (lac::luxCDABE), a constitutively bioluminescent strain, was used to monitor the general toxicity of samples through a decrease in its bioluminescence, while specific toxicity was detected through the use of strains such as DPD2540 (fabA::luxCDABE), TV1061 (grpE::luxCDABE), DPD2794 (recA::luxCDABE), and DPD2511 (katG::luxCDABE). These inducible strains show an increase in bioluminescence under specific stressful conditions, i.e. membrane-, protein-, DNA-, and oxidative-stress, respectively. The toxicity of a sample could be detected by measuring the bioluminescence 30 min after addition to the freeze-dried strains. Using these strains, many different chemicals were tested and characterized. This portable biosensor, with a very simple protocol, can be used for field sample analysis and the monitoring of various water systems on-site.
Recently, biotin was covalently coupled with alginate in an aqueous-phase reaction by means of carbodiimide-mediated activation chemistry to provide a biotin-alginate conjugate for subsequent use in biosensor applications [120]. The synthetic procedure was optimized with respect to pH of the reaction medium (pH 6.0), the degree of uronic acid activation (20%), and the order of addition of the reagents. The new biotin-alginate conjugate was used for the encapsulation of bioluminescent reporter cells into microspheres. A biosensor was prepared by conjugating these biotinylated alginate microspheres to the surface of a streptavidin-coated optical fiber, and the performance of the biosensor was demonstrated in the determination of the antibiotic, mitomycin C as a model toxin.
Also, ionic and colloid gold influence on luminous bacteria Photobacterium phosphorum B7071 bioluminescence have been studied by Gruzina et al. [121]. It was shown that both forms of gold inhibited bioluminescence of the studied bacteria depending on their concentrations and incubation time with cells. The approaches to creation of biosensor systems based on luminous bacteria and semiconductor structures or optical fibers are proposed.
Eltoukhy et al. [122] describes a bioluminescence detection lab-on-chip consisting of a fiber-optic faceplate with immobilized luminescent reporters/probes that is directly coupled to an optical detection and processing system-on-chip fabricated in a 0.18 µm process. The lab-on-chip is customized for such applications as determining gene expression using reporter gene assays, determining intracellular ATP, and sequencing DNA.
On the other hand, hydrophobic phosphorescent Pt-porphyrins have been used for the development of luminescent polymer films designed for fibre-optic oxygen sensors [123]. Luminescent and quenching characteristics of several Pt-porphyrins incorporated into polymer matrices have been studied to optimize the preparation of sensitive coatings for fibre-optical sensors. The films thus obtained have been used for fibre-optical oxygen monitoring in solutions. More recent, a simple system for enzymatic flow-injection analysis of metabolites is described by Ovchinnikov et al. [124], which is based on the phosphorescence lifetime based detection of molecular oxygen using phase-modulation techniques and a simple instrument phosphorescence phase detector equipped with a fibre-optic probe. The phase detector is connected to the oxygen sensor membrane and allows real-time continuous monitoring of the phosphorescence phase shift. This parameter is related to the phosphorescence lifetime of the oxygen probe, therefore giving a measure of the dissolved oxygen concentration, and its changes as a result of the enzymatic oxidative reaction with the substrate. The sensor membrane is positioned in a compact integrated flow-through cell and exposed to the flow stream. Using glucose as a test analyte and glucose oxidase enzyme, two different sensor setups were tested: 1) the membrane type biosensor in which the enzyme is immobilized directly on the oxygen sensor membrane; 2) the microcolumn type biosensor in which the enzyme is immobilized separately, on a microparticle sorbent (controlled pore glass) and put into a microcolumn with the oxygen sensor membrane placed at the column outlet. In either case a new type of oxygen sensitive material was used, which provides a number of advantages over the existing materials. In this material the oxygen-sensitive coating was applied on a microporous scattering support, the latter comprised of a layer of cellulose particles on polyester support.

Refractive index
Akkin et al. [125] describe a fiber-based optical biosensor, which is capable of detecting ultra-small refractive index changes in highly scattering media with high lateral and longitudinal spatial resolution. The system is a dual channel phase-sensitive optical low coherence tomography system that measures relative optical path length differences between the orthogonal modes of the polarization-maintaining fiber.
Tubb et al. [126] describe a new design of optical fibre surface plasma wave chemical sensor. The basic sensor consists of a tapered single-mode optical fibre with a thin layer of silver evaporated onto the tapered section. The gradually changing diameter of the fibre along the taper and the variation in silver depth around the taper result in a distributed coupling between the guided mode of the fibre and the surface plasma wave. As a result, the coupling to the surface plasma wave occurs over an enlarged spectral range. The device shows good sensitivity to refractive index with refractive index changes of 5x10 -4 being detectable.
Also, a biosensor based on long period grating (LPG) technology has been used to demonstrate the detection of large molecules (proteins) and small molecules (pesticides) [127]. The LPG sensor is a spectral loss optical fiber based system that provides direct detection of large molecules, by using an antigen or antibody modified hydrogel, without the need for secondary amplification. The binding of the specific target results in a mass increase that produces a localized refractive index change around the LPG region and thus a spectral shift in the observed wavelength loss band. The magnitude of the observed shift can be correlated to target concentration. The HIV protein p24 was directly detected at 1 ng/mL with a specific signal that was 5-7 times that of the system noise. A direct and indirect competitive assay was demonstrated with the target atrazine. The sensitivity of the two competitive assay formats was in the range of 10-50 ng/mL. The investigation group of Cheng presents a novel class of label-free fiber-optic localized surface plasmon resonance sensor which retains many of the desirable features of the propagating surface plasmon resonance sensors, namely, the sensitivity to the refractive indices of bulk liquids and the ability to interrogate biomolecular interactions without a label [128,129]. The sensor was constructed on the basis of modification of the unclad portion of an optical fiber with self-assembled Au colloids. The sensor is easy to fabricate and can be constructed by simple optical designs. Moreover, the sensor has the potential capability for on-site, in vivo, and remote sensing, can be easily multiplexed to enable high-throughout screening of biomolecular interactions, and has the potential use for disposable sensors.
Optical sensors based on the excitation of surface plasmons (SP) have proven to hold great potential for biomolecular interaction analysis and detection of biological analytes. In order to reach out from centralized laboratories, the surface plasmon resonance (SPR) sensors have to be developed into robust portable sensing devices capable of operating in the field. In this way, an optical fiber SPR sensor based on polarization-maintaining fibers and wavelength modulation is presented by Piliarik et al. [130]. It is demonstrated that this design provides superior immunity to deformation of optical fibers of the sensor and, thus allows for more accurate SPR measurements under realistic operation conditions. Experimental results indicate that this fiber-optic SPR sensor is able to resolve refractive index changes as low as 4x10 -6 under moderate fiber deformations.
The micro-and nano-scale miniaturization of chemical and biochemical sensors is of great scientific and technological interest and in the last decade, much effort was focused on miniaturizing fiber-optic SPR sensors. In this way, a highly sensitive micrometer-sized optical fiber affinity biosensor is reported based on the localized SPR in gold nanoparticles adsorbed at an end-face of an optical fiber; this sensor probes the affinity between biological molecules in real time without any labeling of the analyte; the highest resolution of 10 -5 in refractive index units is demonstrated with a red-light-emitting diode used as a light source [131]; a novel class of fiber-optic biosensor that exploits the localized SPR of self-assembled gold colloids on the grating portion of a long-period fiber grating is proposed [132]; Chang et al. [133,134] present a sensitive nano-optical fiber biosensor made by shaping a fiber to form a taper with a tip size under 100 nm; a 3-D coded finite-difference time-domain approach verifies the excitation of the surface plasmon wave and the differences among its intensities in media of various refractive indices; Lin et al. [135] developed a side-polished multimode fiber sensor based on SPR as the transducing element with a halogen light source; the SPR fiber sensor is side polished until half the core is closed and coated with a 37 nm gold thin film by dc sputtering; the SPR curve on the optical spectrum is described by an optical spectrum analyzer and can sense a range of widths in wavelengths of SPR effects; the measurement system using the halogen light source is constructed for several realtime detections that are carried out for the measurement of the index liquid detections for the sensitivity analysis; the sensing fiber is demonstrated with a series of refractive index liquids and a new type of the fiber-optic microsensor for SPR was created on the basis of the fabrication technology of optical fiber probes in near-field scanning optical microscopy (NSOM) [136]. The newly developed SPR microsensors were prepared by coating a gold-metallic film on the chemically etched single-mode fiber containing a conical core. They were applied to the real-time monitoring of the refractive index (RI) of transparent liquids flowing in the microfluidic device.
In the last years, high sensitivity chemical and biological sensors based on etched core fiber Bragg gratings that detect change in the index of refraction of surrounding solutions, were developed to measure the index of refraction of different solutions [137][138][139][140] and compact three segmented multimode fiber modal interferometer for high sensitivity refractive-index measurement also have been described [141].
On the other hand, fiber-optic waveguides based Micro-Opto-Electro-Mechanical Systems (MOEMS) form a significant class of biosensors which have notable advantages like light weight, low cost and more importantly, the ability to be integrated with bio-systems. Iintegrated microfluidic fiberoptic waveguide biosensor is presented by Chandrasekaran and Packirisamy [142]. The phenomenon of evanescence is employed for sensing mechanism of the device. Herein, the fiber-optic waveguide is integrated with bulk micromachined fluidic channel across which different chemical and biological samples are passed through. The significant refractive index change due to the presence of biological samples that causes the evanescent field condition in the waveguides leads to optical intensity attenuation of the transmitted light. The study of the modulation in optical intensity is used to detect the properties of the species used in the evanescent region. The intensity modulation of light depends upon the geometry of the waveguide, the length of evanescent field, the optical properties of specimen used for producing evanescence and the changes in the properties by their reaction with other specimen. Therefore, this device is proposed for biosensing applications.

Other techniques
Pepper [143] discuss the potential use of nonlinear optical phase conjugation to enhance the performance of various classes of optical interferometric sensors and optical fiber biosensor devices, with potential application to trace-compounds detection of explosives and other species. Examples include Michelson interferometers, laser homodyne sensors, ellipsometers, and modulation spectrometers. Compensated interferometric devices using nonlinear optical phase conjugation may lead to a new class of fieldable remote sensor which is robust, compact, inexpensive, and portable, with the capability of functioning in real-world environments.
An interferometric optical fiber microcantilever beam biosensor has successfully demonstrated real time detection of target molecules [144]. The microcantilever biosensor effectively combines advanced technology from silicon micromachining, optical fiber sensors, and biochemistry to create a novel detection device. This approach utilizes affinity coatings on micromachined cantilever beams to attract target molecules. The presence of the target molecule causes bending in the cantilever beam, which is monitored using an optical displacement system. Dose-response trials have shown measured responses at nanogram ml -1 concentrations of target molecules.
A D-type fiber biosensor based on SPR technology and heterodyne interferometry is presented by Chiu et al. [145]. The sensing device is a single-mode optical fiber in which half the core is polished away and a thin-film layer of gold is deposited.
A fiber-optic sensor is designed based on multicavity Fabry-Perot interferometry for the study of optical thickness in self-assembled thin-film layers [146]. This miniature sensor is applicable not only to the measurement of self-assembled polyelectrolyte layers but also to the immobilization of proteins such as immunoglobulin G.
Recently, a new method of optical switch design is proposed, which is used in optical fiber biological protein chips [147].
Also, in the last year, Rindorf et al. [148] present the first incorporation of a microstructured optical fiber (MOF) into biochip applications. A 16-mm-long piece of MOF is incorporated into an opticfluidic coupler chip, which is fabricated in PMMA polymer using a CO 2 laser. The developed chip configuration allows the continuous control of liquid flow through the MOF and simultaneous optical characterization. While integrated in the chip, the MOF is functionalized towards the capture of a specific single-stranded DNA string by immobilizing a sensing layer on the microstructured internal surfaces of the fiber. The sensing layer contains the DNA string complementary to the target DNA sequence and thus operates through the highly selective DNA hybridization process. Optical detection of the captured DNA was carried out using the evanescent-wave-sensing principle. Owing to the small size of the chip, the presented technique allows for analysis of sample volumes down to 300 nL and the fabrication of miniaturized portable devices.
Such as we described above, there is an increasing demand for sensitive detection systems which are required to detect analytes in often very dilute and diverse circumstances. Fiber-optic biosensors have recently gained a lot of interest, and different schemes have been proposed and applied to detect a wide variety of analytes. The antibody-antigen or enzyme-analyte recognition or reaction provides the basis for the sensitivity and/or selectivity of the reaction. The transduction of the biochemical signal to an electrical signal is often a critical step wherein a large fraction of the "signal loss", for example, fluorescence (by quenching) may occur. This leads to deleterious effects on the sensitivity and selectivity of the biosensor, besides decreasing the quality of the reproducibility of the biosensor. A fractal analysis is presented for the binding of pyrene in solution to β-cyclodextrin attached to a fiberoptic chemical sensor [149]. The fractal analysis provides novel physical insights into the reactions occurring on the fiber-optic chemical surface and should assist in the design of fiber-optic chemical sensors [150,151].
Noto et al. [152] report on molecular weight dependence measurements for an optical resonance biosensor. A dielectric micro-particle is evanescently coupled with an optical fiber for the resonance stimulation, and a shift of the resonance wavelength is measured to monitor protein monolayer formation on the micro-particle surface. Wavelength shifts for proteins over two orders of magnitude in molecular weight are measured.
Walt use optical imaging fibers to fabricate a chemical and biochemical sensor that utilizes the ability of living cells to respond to biologically significant compounds. The sensor is created by randomly dispersing single NIH 3T3 mouse fibroblast cells into an optically addressable fiber-optic microwell array such that each microwell accommodates a single cell. The cells are encoded to identify their location within the array and to correlate changes or manipulations in the local environment to responses of specific cell types [153,154].
The fabrication and testing of a novel waveguide based biosensor for sensing in microchannels is presented by Deverkadra and McShane [155]. Unlike evanescent wave sensing, which has large intrinsic losses, an absorption based sensing scheme was introduced. The fabricated waveguide sensor has excellent transparency in the UV-Vis region of the spectra and is able to perform simultaneous detection of multiple analytes in microchannels. The optical interconnection to the device was achieved using self-aligned V-grooves etched on the silicon wafer using KOH etching. The fabricated device has applications in micro total analysis systems for measuring various analytes in the microchannels, tissue engineering for continuous measurement of the oxygen in the artificial scaffolds, and fluorescencebased measurement by attaching fluorescent dyes to the waveguide end face in the microchannel.
Finally, Konry et al. [156,157] demonstrate that it is possible to create surface-conductive fiber optics, upon which may be electropolymerized a biotinylated polypyrrole thin film, which may then be used to affinity coat the fiber with molecular recognition probes. This fiber-optic electroconductive surface modification is done by the deposition of a thin layer of indium tin oxide. Thereafter, biotinpyrrole monomers are electropolymerized onto the conductive metal oxide surface and then exposed to avidin. Avidin biotin interactions were used to modify the fiber optics with biotin-conjugated cholera toxin B subunit molecules, for the construction of an immunosensor to detect cholera antitoxin antibodies.

Applications
Optical fibers have been used for a variety of sensing applications. The small physical dimensions and the ease with which they can be used for multiplexed analyte detection make them ideal platforms for sensing. Bioanalytical microsystems based on miniaturized biosensing elements could find wide applications in DNA analysis, drug discovery, medical diagnostics, and environmental monitoring as well as in protection against bioterrorism. Miniaturization, portability, multianalyte potential, and interfacing with electronic functions are critical elements for biosensing devices to meet the demands in these fields.
Optical fibers have been used to develop sensors based on nucleic acids and cells. Sensors employing DNA probes have been developed for various genomics applications and microbial pathogen detection. Live cell-based sensors have enabled the monitoring of environmental toxins, and have been used for fundamental studies on populations of individual cells [158].
Advances in nanotechnology have recently led to the development of fiber optics-based nanosensor systems having nanoscale dimensions suitable for intracellular measurements. The possibilities to monitor in vivo processes within living cells could dramatically improve our understanding of cellular function, thereby revolutionizing cell biology. Fiber optic sensors provide significant advantages for in situ monitoring applications due to the optical nature of the excitation and detection modalities. Fiber optics sensors are not affected by electromagnetic interferences from static electricity, strong magnetic fields, or surface potentials. Another advantage of fiber optic sensors is the small size of optical fibers, which allow sensing intracellular/intercellular physiological and biological parameters in microenvironments. Biosensors, which use biological probes coupled to a transducer, have been developed during the last two decades for environmental, industrial, and biomedical diagnostics. Nanotechnology has been revolutionizing important areas in molecular biology, especially diagnostics and therapy at the molecular and cellular level. The combination of nanotechnology, biology, and photonics opens the possibility of detecting and manipulating atoms and molecules using nanodevices, which have the potential for a wide variety of medical uses at the cellular level. The nanoprobes were fabricated with optical fibers pulled down to tips with distal ends having sizes of approximately 30-50 nm. The nanoscale size of this new class of sensors, allows for measurements in the smallest of environments. One such environment that has evoked a great deal of interest is that of individual cells. Using these nanobiosensors, it has become possible to probe individual chemical species in specific locations throughout a cell [159,160].
Biosensors utilizing immobilized antibodies have become increasingly useful due to the specificity of antigen-antibody interactions and the fact that the biosensors can be used where typical cuvettebased spectroscopic measurements are not practical. Of particular usefulness are fiber optic-based biosensors, as the fiber upon which the antibody is immobilized serves as a conduit for measuring optical changes occurring during the antigen-antibody binding event.
Fluorescence-based biosensors provide advantages over other forms of biosensors in that the high signal-to-background discrimination associated with fluorescence is coupled to the antigen binding event. Biosensors utilizing fluorescence may also take advantage of evanescent wave excitation, where excitation light propagating through the fiber excites fluorophores within close proximity (.100 nm) to the fiber surface. Excitation via this technique results in fluorescencefrom labeled antigens only when the antigensbound by antibodies are within the evanescent wave. Thus, background signals from bulk solution contribute little or nothing to the total measured fluorescence, in contrast to cuvette-based measurements where signal from unbound antigen may dominate. Evanescentwave biosensors are also extremely useful for conducting analyses in opaque media, heterogeneous solutions, or colloidal suspensions.
One continuing problem of using fluorescence-based fiber optic biosensors is that of variations in signal response among individual fibers. Wadkins et al. [161] addressed this problem by labeling a portion of the immobilized capture antibody with the fluorescent cyanine dye Cy5.5 (emission λ max = 696 nm). The antigen was then labeled with fluorescent Cy5 (emission λ max = 668 nm). Both fluorophores were excited by 635 nm light, and their emission was collected using both a fiber optic spectrometer and a biosensor optimized to collect fluorescence at two wavelengths. The fluorescence from the Cy5.5-labeled capture antibody served as a calibration signal for each fiber and corrected for differences in optics, fiber defects, and varying amounts of capture antibody present on the fiber. Also, Alexa Fluor 647 (AF647) is a relatively new fluorescent dye, which has peak excitation at 650 nm and peak emission at 665 nm. It has a molecular weight of aproximately 1300. These properties are similar to the more commonly employed dye Cy5. On the other hand, the RAPTOR is a portable, automated biosensor that, like its predecessor the Analyte 2000, is useful for on-site analysis of food, water, or clinical samples for biological contaminants. The performance of the AF647 was explored as an alternative to Cy5 for immunoassays on the RAPTOR. The RAPTOR performs sandwich fluoroimmunoassays on the surface of small polystyrene optical waveguides for analyte detection. Primarily, due to the self-quenching characteristics of Cy5, AF647 is substantially more effective in fluoroimmunoassays, yielding over twice the signal for any given analyte concentration. The limitations of Cy5 were elucidated with an immunoassay for ricin, while the advantages of AF647 were demonstrated in both direct binding assays as well as in a sandwich immunoassay for staphylococcal enterotoxin B [162]. The principal applications of these biosensors are summarized in table 2.

Conclusions
Fiber-optic biosensors will play a significant role in the development of biosensors because they can be easily miniaturized and integrated for the determination of different target compounds. These biosensor types have been the objective of a large number of investigations in the last years and they provide numerous ways of performing the rapid, remote, in-line and on-line determination of a lot types of analytes in a wide range of application fields. They are under continuous development and research in this area places increasing emphasis on the works concerning the sensors' performance, such as micro-structural stability, leaching, reversibility, response time, repeatability, sensitivity and selectivity, instead of simply demonstrating the sensing potential. Also, rapid advances have been made in improving immobilization protocols.
Likewise, one of the major advantages of using optical biosensors in conjunction with optical fibers is that it permits sample analysis to be done over long distances and this has important implications for field monitoring. However, the main drawback apart from being relatively expensive is that optical fibers may suffer from miniaturization problems. Notwithstanding, the application of optical fiberbased nanosensors has become an area of significant interest and various methods have been developed to alleviate the problems arising from miniaturization. In this sense, optical fiber SPR probes present the highest level of miniaturization of SPR devices, allowing for chemical and biological sensing in inaccessible locations where the mechanical flexibility and the ability to transmit optical signals over a long distance make the use of optical fibers very attractive.
In summary, fiber-optic biosensors will play a significant role in the development of biosensors because they can be easily miniaturized and integrated for the determination of different target compounds in a wide variety of application fields, such as industrial process and environmental monitoring, food processing, and clinical applications.