A Green Slab Waveguide for Plasmonic Sensors Based on Bacterial Cellulose

: We use as optical waveguide a green composite, based on bacterial cellulose (BC). More specifically, we have sputtered a thin gold film on this innovative slab waveguide for obtaining a Localized Surface Plasmon Resonance (LSPR) sensor. Experimental results confirm the possibility of using the BC based composite as an environmental friendly optical sensor platform with plasmonic capabilities, which could be exploited for realizing disposable biosensors. The new optical sensor has been used by combining it with optical fibers. The fibers connect the green disposable optical sensor with a light source and with a spectrometer. The device has been tested by measuring the refractive index of different water-glycerin solutions.


Introduction
Surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) bio-chemical sensors in optical fibers are suitable for on-site and real-time monitoring of different analytes. As such, they play an important role in many research fields [1][2][3][4][5]. Many solutions have been proposed in order to optimize the performances of the SPR/LSPR sensors in terms of throughputs, reliability, robustness and miniaturization [6][7][8][9][10][11]. The optical fiber sensors can be defined as intrinsic and extrinsic, according to the interaction of the fiber with the analyzed medium (intrinsic) or its use as a mere waveguide allowing the launch of the light to the sensing region and its collection (extrinsic). The sensing scheme may be founded on reflection, where the light source and the detector lay on the same side of the fiber, or on transmission, where they are on opposite sides.
In this work, we show a transmission extrinsic optical fiber sensor, based on a slab waveguide of bacterial cellulose (BC), covered by a gold film. Cellulose is the most ambulant organic polymer present in nature. It is biocompatible and fully biodegradable [12]. Eventually, it can be processed to be used in "non conventional" flexible and green electronics. Cellulose is, generally, obtained by pant sources by the pulp industry. Unfortunately, the extraction of cellulose requires relevant amounts of both energy and resources (fresh water). Eventually, waste products are obtained, which can cause pollution, if released in the environment [13]. Because of the production process, the appeal of cellulose as a candidate for realizing greener sensing systems is greatly reduced. Though BC shares with plant-derived cellulose the same chemical structure, it is obtained by totally different sources. BC is produced, as a matter of fact, by some bacteria, in suitable cultures [14]. The culturing of BC is possible in typical laboratory environmental conditions. Moreover, no relevant quantities of energy and water are required. Because of the described production procedure required for BC, it is considered a greener alternative to plant-derived cellulose. Not less important, BC is produced in a much purer form than cellulose derived from plant sources.
As a consequence of the considerations reported above, a growing interest has been devoted to BC as a base component for realizing green transducers. Recently, some of the authors, have shown the possibility of using BC for realizing mechano-electric transducers [15][16].
Here, the possibility of exploiting BC based compounds for realizing optical sensors is addressed. A plasmonic sensor exploiting a BC thin film, with embedded silver nanoparticles has already been presented by NahidPourreza et al. [17], but their approach (a BC nanopaper, with silver nanoparticles) differs from the one proposed in this paper mainly because the metal (gold in our case) covers the cellulose wires so configuring a kind of nanowires able to excite LSPR.

Plasmonic Sensor System
In this work, we have covered by a thin gold film (60 nm) a BC-based composite, similar to that already used as an accelerometer [16]. Figure 1a shows the SEM image of the used bacterial cellulose based compound.
The procedure for the fabrication of the plasmonic platform includes only one step: gold is sputtered on the top of the CB-based paper, by using a sputtering machine (Bal-Tec SCD 500). In particular, the sputtering was repeated three times with a current of 60 mA for 35 s (20 nm per step). The green plasmonic sensor platform has been characterized using a simple experimental configuration. The setup is based on a halogen lamp (HL-2000-LL, Ocean Optics), an optical coupler (50:50), two optical fibers connected with two similar spectrometers (USB2000+UV-VIS spectrometer, Ocean Optics), and two green slab waveguides of bacterial cellulose, one covered by gold (sensor) and one without gold (reference). Figure 1b shows the experimental setup.
The spectral emission of the lamp ranges from 360 nm to 1700 nm and the spectrometer is sensitive from 300 nm to 1050 nm. The measurements have been performed by using different water-glycerin solutions in contact with the plasmonic slab waveguide. In Figure 2 are presented the experimentally obtained LSPR transmission spectra, normalized to the spectra achieved by the reference slab waveguide with the same surrounding medium, for three different water-glycerin solutions, with refractive index ranging from 1.332 to 1.350.

Conclusions
The proposed analysis shows that the chosen green slab waveguide, covered by gold, can be used to make environmental friendly LSPR sensor platforms, useful to realize disposable biosensors. Preliminary experimental results, obtained with different water-glycerin solutions, indicate that the sensor's response to refractive index changes is satisfactory.