Phthalocyanine-Modified Electrodes Used in the Electroanalysis of Monoamine Neurotransmitters

Abstract

Metallo-phthalocyanines (MPcs) are versatile materials with applications in electroanalysis because of their superior catalytic properties. This review presents the electrochemical methods based on MPc-modified electrodes and reports some of their remarkable properties and applications in the electroanalysis of monoamine neurotransmitters and biomolecules that play a crucial role in vital functions of the human body. The development of electrocatalytic chemically modified electrodes is based on their ability to provide a selective and rapid response toward a specific analyte in complex media without the need for sample pretreatment. The explanation of several phenomena occurring at the MPc-modified electrode surface (e.g., MPc-mediated electrocatalysis), the advantages of promoting different electron transfer reactions, and the detection mechanism are also presented. The types of MPcs and different materials, such as carbon nanotubes and graphene, used as substrates for modified working electrodes are discussed. Modifying the properties of MPcs through various interactions, or combining MPcs with carbonaceous materials, creates a synergistic effect. Such hybrid materials present both extraordinary catalytic and increased conductivity properties. We conducted a compilation study based on recent works to demonstrate the efficacy of the developed sensors and methods in sensing monoamine neurotransmitters. We emphasize the analyte type, optimized experimental parameters, working range, limits of detection and quantification, and application to real samples. MPc–carbon hybrids have led to the development of sensors with superior sensitivity and improved selectivity, enabling the detection of analytes at lower concentrations. We highlight the main advantages and drawbacks of the discussed methods. This review summarizes recent progress in the development and application of metallo-phthalocyanine-modified electrodes in the electroanalysis of monoamine neurotransmitters. Some possible future trends are highlighted.

1. Introduction

Electrochemical detection is an alternative to classical chemical analysis and has attracted attention due to its simplicity, along with a minimal sample preparation, rapid response, low-cost instruments and an affordable cost per sample analysis [1]. Modern electrochemical methods are sensitive, selective and easily applicable in the electroanalysis of different compounds. These characteristics make them a versatile tool for a variety of applications.

Electrochemical devices offer specific properties to address the challenges of analytical chemistry, including the simultaneous determination of analytes with similar structures. Of these, electrochemical sensors hold an important position among the available detection systems being widely used in electro-analytical applications. In voltammetric detection, the current signal is generated as a function of an analyte redox reaction at the surface of the working electrode. The potential applied between the working and reference electrodes represent the driving force for the electron transfer reaction of the electroactive species. Electrons crossing the electrode–solution interface can be determined by measuring the current, which is a direct measure of the rate of the electron transfer reaction and is proportional to the concentration of the target analyte [2].

Electrochemical sensors utilize modifiers such as metallo-phthalocyanines (MPcs), metal Schiff bases and metallo-porphyrins, which have been used in the design of chemically modified electrodes (CMEs). The modifiers introduce some new chemical and electrochemical properties that the bare electrodes do not have [3]. Such electrodes have allowed the study of various chemical compounds that, on conventional electrodes, exhibit a very slow heterogeneous electron transfer rate and, consequently, irreversible electrochemical behavior. The primary advantages of MPc modifiers include the increase in the electrode response toward new analytes that are either poorly or not at all electrolyzed at bare electrodes, the major control of electrode selectivity and surfaces that are more resistant to electrode fouling [2].

By using MPcs in conjunction with carbonaceous materials, like carbon nanotubes or graphene, it is possible to produce hybrid materials. These MPc hybrids have shown tremendous advantages in electrochemical sensor design. Carbonaceous materials substantially enhance the electrical conductivity and surface area of sensors, allowing improved detection. Furthermore, they increase the sensing performance through their catalytic activity and signal amplification. The nanomaterials add structure support and a more active surface area, while the MPcs improve the electron transfer processes. These synergistic effects can lead to enhanced performance in applications like catalysis and sensing.

2. Metallo-Phthalocyanines

MPcs are a class of compounds with four fused benzo rings containing an 18 π electron conjugated system in which diverse metals and non-metals can be included (Figure 1). The incorporation of different ring substituents in the peripheral and non-peripheral positions is possible [4].

They have applications in the paint industry [5], photodynamic therapy [6], optical sensing [7], optical computer rewritable discs and information storage systems [8], liquid crystal display devices [9], photovoltaic cells [10], fuel cells [11], semiconductor devices [12] and electrochemical sensors [13,14,15,16,17,18]. Many analytes of biological and environmental relevance, including pesticides [19], nitric oxide [20], thiols [21], hydrazine [22] and phenols [23], were detected by MPc-modified electrodes.

2.1. Metallo-Phthalocyanine Properties

MPcs are involved in the fast redox reactions of a plethora of analytes and play the role of electron mediators or electrocatalysts in the electron transfer process at the electrode–solution interface. A major factor contributing to this use is the physical and chemical stability of MPcs during the oxido-reduction reactions.

The reactive centers of MPcs are clearly identified and their reactivity can be modulated by changing the nature of the central metal or peripheral substituents on the phthalocyanine ring. MPcs have rich redox behavior and the well-defined oxidation–reduction peaks can be attributed to the phthalocyanine (Pc) ligand and the metal atoms included in the structure of MPcs. Redox properties of MPcs are correlated with their electrocatalytic characteristics. In the neutral state, the Pc ring has an oxidation state of −2 (Pc²⁻) [24]. The Pc ring can be oxidized by the successive elimination of up to two electrons and reduced by the successive acceptance of up to four electrons [25,26]. The redox potential of MPcs can be easily modulated by introducing the appropriate substituents on peripheral or non-peripheral positions of the phthalocyanine ligand. In this way the catalytic activity of MPcs for the oxidation or reduction of analytes can be “tuned” by manipulating the formal potential by using the proper groups on the ligand. The central metal ion in MPcs also contribute to tuning the catalytic properties. Among the MPcs, transition metals, such as Co(II), Fe(II), Fe(III) Cu(II), Ni(II) and Mn(II), are commonly used as metal centers in Pcs and assume the role of modifiers (electrocatalysts). Metal oxidation or reduction occurs at potentials between those corresponding to ligand reduction or oxidation. The metal atom at the center of MPcs can undergo reversible redox reactions. Moreover, the MPc central metal atom can be customized to associate specifically with explicit analytes. Copper phthalocyanine has a high affinity for nitrogen-containing compounds, while cobalt phthalocyanines are known for their catalytic activity in oxygen reduction reactions. MPcs have wide working potentials in both anodic and cathodic directions, which allows for the oxidation and reduction of analytes in a large potential window. The redox potentials of the central metal ion in MPcs can be tuned by the choice of the central metal or by the surrounding ligands, leading to synergistic effects in electrocatalysis for different reactions.

The redox properties and catalytic activity of MPcs can be modified by changing the central metal, its oxidation state, and the nature of the axial ligands and ring substituents (substituents at the alpha (α) and beta (β) positions) [27]. As electrocatalysts, the majority of metallo-phthalocyanine complexes involve a change in the oxidation state of the central metal. However, few MPcs have shown ring-based catalysis instead of the central atom catalysis, i.e., the central metal is inactive.

Some MPcs have been studied in both homogeneous and heterogeneous phases as active catalysts in electron transfer reactions involving a great number of molecules [28]. They catalyze the electrooxidation of oxalic acid [29], glucose [30], H₂O₂ [31], hydrazine [32], mercaptoethanol [33], L-cysteine [34], hydroxylamine [35], dopamine [36,37,38] and phenols [39,40], and the electroreduction of H₂O₂ [41], alkyl chlorides [42] and cystine [43]. They have been found to enhance the response for solutes whose direct oxidation or reduction is not kinetically favored. Moreover, MPcs have the capacity to mimic the catalytic activity of certain enzymes [44,45,46].

2.2. Electrocatalysis

MPcs are known to mediate/electrocatalyze the redox process at the central metal [47,48,49] and/or ring [50], facilitating the electron transfer of different analytes. The mechanism for the electrocatalytic oxidation process due to the metal center and that for the one due to the ring are described elsewhere [51]. The electrocatalytic effect is evidenced by a shift in the electrode reaction overpotential to a lower value, an increase in the current or both.

Figure 2 shows an electrocatalytic reduction process at the surface of an MPc-based electrode. The MPcs do not participate in the overall reaction, but its role is to shuttle electrons between the analyte and the electrode surface. During the electrocatalytic reduction process, MPc changes its oxidation state by interacting with the analyte and returns to its initial state by donating electrons to the electrode. Electrocatalytic activities are usually observed at potentials close to the formal potential of the MPc.

The use of chemical modifiers (mediators/electrocatalysts) has resulted in better-performing sensors that exhibit increased electron transfer kinetics and electrocatalytic activity, better sensitivity and improved selectivity toward analytes by reducing the influence of the interferents on the electrode response [52].

3. Metallo-Phthalocyanine-Modified Electrodes

Modifying the electrode surface transfers the modifier’s new physicochemical properties to the electrode, providing molecular-level control over the fabrication of the sensing interface. Electrocatalysis of an analyte–electrode reaction represents an important reason for surface modification, which improves the chemical and electrochemical properties of the electrodes. Based on their good electrocatalytic properties, MPc-modified electrodes can reduce the residual current and consequently increase the current response, lowering the detection limits, improving the selectivity and reducing the analysis time.

The only drawback of MPcs used in sensor design is related to the fact that they exhibit semiconducting properties [53]. As a result, new hybrid materials have been developed that combine MPcs with a highly conductive material (e.g., carbon nanotubes). The two modifiers (mediator and carbon nanotubes) show a synergistic effect when co-used to modify the electrode by further lowering the detection potential and increasing the generated currents [54]. The MPc-modified electrodes provide an alternative way to prepare electrochemical sensors that reveal new types of transduction mechanisms [55,56,57,58,59,60,61,62,63,64].

3.1. Carbonaceous Materials for Electrode Modification

Carbon-based nanomaterials have received great attention in research and application in diverse fields. They exhibit a large surface area, biocompatibility, chemical and electrochemical stability, and good electrical conductivity, and are also inexpensive [65]. Such nanomaterials include carbon nanotubes [66], graphene oxide nanosheets [67] and fullerenes (carbon balls) [68]. Graphite is the main source of nanosized carbon materials. Graphite can be oxidized and exfoliated to form graphene oxide nanosheets, which, in turn, can be folded into different shapes, resulting in carbon nanotubes and fullerenes.

3.1.1. Carbon Nanotubes

Carbon nanotubes (CNTs) were discovered by Iijima and they were produced from a cathode by a carbon arc discharge method [69]. They are molecular-scale tubes of graphitic carbon and have become attractive materials for a wide range of applications, including the field of electrochemical sensors [70,71]. This is mainly due to their unique structural, electronic and mechanical properties. These 2D, rigid and strong fibers present a closed topology and tubular structure, along with a hollow core suitable for storing guest molecules. Their large length-to-diameter ratios result in high surface-to-volume ratios.

There are two categories of carbon nanotubes: single-wall (SW) and multi-wall (MW) carbon nanotubes [72]. SWCNTs have a cylindrical nanostructure formed by rolling up a single sheet of graphite (graphene) into a tube with either open or closed ends, depending on the fabrication methodology [73]. The closure of the cylinder is due to pentagonal inclusion in the hexagonal carbon network of the nanotube walls during the growth process. SWCNTs typically have diameters of ~1 nm, with the smallest diameter reported to date of 0.4 nm [74]. The structure of individual graphene tubes is defined by the unit cell, where the open ends may have “zig-zag”, “arm-chair” and “chiral” geometries [75]. MWCNTs are coaxial assemblies of concentrically nested graphene cylinders with an interlayer spacing of 0.34 nm [76], with dimensions ranging from 2 to 30 nm in diameter and several microns in length [77].

The electroanalytical applications of CNTs stem from their excellent electroactivity, resulting in electrocatalytic behavior, and the ability to promote and mediate fast electron transfer kinetics in electrochemical reactions involving a wide range of electroactive species when CNTs are used as electrode materials [78]. CNT-modified electrodes improve electron transfer compared with their unmodified counterparts [79,80,81,82,83]. The origin of electron transfer for CNTs is at the open end of the nanotubes and along the tube axis, where defect sites exist and at which electron transfer is reported to be faster than that of the pristine sidewalls of the nanotubes [84,85].

From the perspective of the electrochemical reactivity, the sidewalls of CNTs are suggested to have properties similar to those of the basal plane of highly oriented pyrolytic graphite (HOPG), while their open ends resemble the edge planes of HOPG. Accordingly, electron transfer rates similar to those of a graphite edge plane electrode can be expected. The open end of an MWCNT has a fast electron transfer rate comparable with that of a graphite edge plane electrode, while an SWCNT presents a very slow electron transfer rate and a low specific capacitance, equivalent to that of the graphite basal plane [86].

The electrocatalytic activity of CNTs is associated with their dimensions, electronic structure and topological defects present on the tube surface [87,88]. The presence of pentagonal defects produces regions of higher charge density than those observed in the region of hexagonal graphite, either in planar or in tubular structures, demonstrating the relationship between topological defects and the electroactivity of CNTs [87]. Compton’s group assumed that this enhanced electrocatalytic activity was due to the edge-plane-like sites located at the ends and in the “defects” areas of the tubes [88]. Moreover, it has been shown that the electrocatalytic activity of MWCNTs strongly depends on the CNT fabrication method and the dispersing agent used to immobilize the CNTs on the electrode surface [89]. The oxygenated species residing at the edge-plane-like sites (defects) of the carbon nanotubes are also considered; it is well known that electrochemically active species may interact favorably or detrimentally with surface oxygenated species [84].

Carbon Nanotubes as Electrode Material

Before application in electroanalytical assays, the synthesized CNTs can be treated to functionalize their surfaces. The functionalization and surface immobilization of CNTs represent a critical step in sensor design. Chemical functionalization of CNTs was used to attach the desired chemical compounds to them. End [90] and/or sidewall [91] functionalization, use of surfactants with sonication [92], polymer wrapping of nanotubes [93] and protonation by superacids [94] have been used for this purpose. The preparation of homogeneous dispersions of CNTs suitable for processing into thin films is of great importance [74]. This has permitted the realization of composite electrodes consisting of CNTs well-dispersed in a suitable polymer matrix [95].

In general, some pre-treatment of the CNTs is required to remove metallic impurities, and/or to improve the electron transfer properties and/or to allow further functionalization. CNT walls are not reactive, but their fullerene-like tips are known to be more reactive, so the end-functionalization of CNTs is used to generate functional groups (e.g., −COOH, −OH or −C=O). The protocols are based on the oxidation of CNTs under different conditions. Treatment with strong acids removes the end caps; may shorten the length of the CNTs; and also adds oxide groups, primarily carboxylic acids, and phenolic groups to the tube ends and defect sites. These functional groups provide sites for the covalent bonding of CNTs to biorecognition elements (or other materials) or for their integration onto polymer surface structures. These oxide groups can undergo further chemical reactions, allowing for functionalization with groups like amides and thiols. Activation by treatment in acidic solutions has been widely used. Solutions of sulfuric [81], nitric [96] and hydrochloric [97] acids, concentrated or diluted, alone or mixed [96], have been used at room temperature.

Electrochemical treatments have also been employed, and in some cases, they were based on a combination of different chemical and electrochemical protocols [98].

The non-covalent functionalization of CNTs by small molecules can alter the electrochemical properties of the material [99]. This type of functionalization is based on weak interactions and can be used to attach small molecules or biochemically active molecules. The CNT solubilization by covalent modification was reported by Luong et al. [100]. Other derivatization methods include defect functionalization [101].

Lower overvoltage and higher peak currents are observed in the voltammetric response of several molecules at CNT-modified electrodes. Due to these unique properties, CNTs have received enormous attention for the preparation of electrochemical sensors, as was extensively reviewed [102,103,104,105,106,107].

Carbon Nanotube Paste Electrodes

By analogy with carbon paste and composite electrodes, similar matrices that involve CNTs have been used in the construction of electrochemical sensors. CNTs can be distributed within the polymer matrix in either a random or vertical orientation. A variety of binders (e.g., mineral oil, Teflon or epoxy resins) were used to produce CNT pastes or composites. CNT paste electrodes have been obtained by mixing CNT powder with deionized water [108], bromoform [109] or mineral oil [110]. The high surface area of CNTs allows the construction of stable, robust paste electrodes with high amounts of mineral oil (50% CNTs and 50% mineral oil).

3.1.2. Graphene

In 2004, researchers Geim and Novoselov discovered and isolated graphene, a two-dimensional atomic crystal with a hexagonal lattice of carbon atoms [111]. Some authors describe graphene as a material “which consists of a single atomic sheet of conjugated sp² carbon atoms” [112]. The International Union of Pure and Applied Chemistry (IUPAC) defines it as “a single carbon layer of the graphite structure, describing its nature by analogy to a polycyclic aromatic hydrocarbon of quasi-infinite size” [113]. Graphene is the basic structure of all graphitic materials and is a one-atom-thick planar sheet of sp²-bonded carbon atoms in a rigid honeycomb lattice [114]. It resembles a large polyaromatic molecule of semi-infinite size, emerging as a true two-dimensional nanomaterial.

Graphene has excellent properties, such as a high surface-to-volume ratio, high thermal conductivity [115], high electron transfer rates and extraordinary electron mobility [116]. Its thermal stability, robust mechanical strength and flexibility, pliability, impermeability and low energy dynamics of electrons with atomic thickness, as well as good biocompatibility [117], make graphene ideal for fabricating sensitive electrochemical devices. Graphene is an excellent conductor of electrical charge, and heterogeneous electron transfer occurs at its edges or at defects in the basal plane [118]. Furthermore, the high surface area of graphene facilitates the presence of large amounts of defects and electroactive sites [119].

Graphene-based electrodes have shown superior performance in terms of electrocatalytic activity and macroscopic scale conductivity compared with CNT-based ones [120,121]. The main drawback of using CNTs for electrode modification is the presence of electrochemically active metallic impurities [122,123] that can influence the electrochemistry of CNTs. Since graphene does not contain metallic impurities, it is gradually competing with CNTs in many applications [124,125]. Another reason for the considerable interest in graphene is its easy availability.

3.2. Methods of Electrode Modification

Various techniques can be used to modify electrodes with MPcs, which include covalent bonding [126], adsorption (dip drying [127] and drop drying [128]), spin coating [129], electropolymerization [130], the Langmuir–Blodgett technique [131], vapor deposition [132], self-assembled monolayers [133], click chemistry [134] and electrografting [135].

The method of covalent bonding is complicated and involves functional groups of CNTs, such as −COOH or −NH₂, which are generally reacted with MPcs. Amino-substituted MPcs complexes can be covalently linked to CNTs via amide bond formation [126].

Non-substituted MPc complexes are adsorbed onto carbon materials via π–π interactions by immersing the electrode in a solution of the complexes for several minutes or placing a drop of a solution containing a known amount of the complexes on the electrode surface. This is a simple and rapid method of electrode modification. This method was chosen because MPcs have a readily available π electron system that can interact with the π electron system of the carbon electrodes, thereby forming a relatively stable π-stacked catalyst film on the electrode surface.

Drop drying is preferable to dip drying because it is possible to control the amount of modifier adsorbed to a certain degree, thereby increasing the reproducibility of the experimental conditions and results. The drop-drying technique is used to deposit the suspension solution of the composite MPc–MWCNT material to the main electrode. It has been observed that phthalocyanine–CNTs complexes have the excellent catalytic properties of phthalocyanines without losing any of the electronic properties of carbon nanotubes [136].

More stable electrodes can be achieved by the electrodeposition of MPc complexes on the electrode surface [13,130,137].

4. Analytical Applications of MPc-Modified Electrodes for Detection of Monoamine Neurotransmitters

Neurotransmitters are the primary biochemical messengers of the central and peripheral nervous systems. As electrochemical signaling molecules, neurotransmitters are essential for proper brain function, including the perception, behavior, motor control, cognitive behavior and emotional states of an organism. They affect and adjust the muscle tone and heart rate, and regulate learning, sleeping, memory, consciousness, mood and appetite [138]. These neurotransmitters are essential for human health and any imbalance in their activities can cause serious mental disorders.

There are more than 100 known neurotransmitter molecules [139]. Neurotransmitters have been classified as either excitatory or inhibitory based on their behavior between two neurons and as amino acids, peptides, monoamines, gas transmitters, trace amines or purines according to their chemical structure. Those with a single amino functional group are classified as monoamine neurotransmitters (Figure 3) [140] and include serotonin (5-hydroxytryptamine, 5-HT) and catecholamines: dopamine (DA), epinephrine (EP) and norepinephrine (NP).

Changes in the concentrations of neurotransmitters in the central nervous system have been associated with many mental and physical disorders, including Parkinson’s disease, Alzheimer’s disease, schizophrenia, glaucoma, Huntington’s disease, epilepsy, arrhythmias, thyroid hormone deficiency, congestive heart failure, sudden infant death syndrome, addiction, Tourette’s syndrome, depression and anxiety [141,142,143,144,145,146,147]. It is essential to monitor the concentrations of monoamine neurotransmitters when studying and diagnosing these disorders.

Hence, the detection of neurotransmitters and measurement of their activity in biological samples is of fundamental importance in both basic neurophysiology and clinical neurology research. It is vital in the diagnosis and treatment of brain diseases.

Electrochemical techniques have been identified as simple, inexpensive and less time-consuming. Electrochemical analysis is based on the redox behaviors of monoamine neurotransmitters, as well as of their metabolites. Various electroanalytical techniques have been employed to detect and monitor neurotransmitters, such as cyclic voltammetry, differential pulse voltammetry, square wave voltammetry and amperometry. These electrochemical sensing strategies have the potential to achieve the rapid, sensitive, selective, and low-cost detection of biomolecules relevant to clinical diagnosis and treatment monitoring.

4.1. Determination of Dopamine

Dopamine (3, 4-dihydroxyphenyl ethylamine or [2,(3,4-dihydroxyphenyl) ethylamine]) exists in the brain and central nervous system of mammals. It is both an inhibitory and an excitatory neurotransmitter and it is partially responsible for motor activity, as well as the physiological regulation of behavior, mood, memory, perception and learning [148]. DA regulates cardiovascular, hormonal, renal and gastrointestinal functions, among others [149,150,151,152,153]. Disruptions to the DA balance in the human body have been associated with conditions such as Parkinson’s disease, dementia, epilepsy, schizophrenia and melancholia [154]. Increased dopaminergic activity may result in perceptual errors, like hallucinations, and is a characteristic of certain psychiatric disorders, such as schizophrenia. On the other hand, a reduction in dopaminergic neurons is often seen in certain neurological and motor illnesses, such as Parkinson’s disease. DA is essential for processing rewards, regulating emotional states, and influencing human motivation and goal-directed behavior [155]. Therefore, the detection and quantification of DA are significant for disease diagnosis.

Table 1. Electrochemical sensors based on phthalocyanines reported for DA detection.

Analytes	Electrode and Medium	Detection Technique	Linear Range [×10−6 mol·L−1]	Detection Limit [×10−6 mol·L−1]	Sample	Recovery (%)	Ref.
DA	NiTAPc/GCE 0.1 M PBS, pH 7.4	LSV	0.2–20	0.09	Pharmaceuticals Plasma Urine	96.5–103.0 97.3–103.5 95.7–103.6	[38]
DA	MWCNT/CoPc/GCE 0.1 PBS, pH 4.0	DPV	3.11–93.25	0.256	-	-	[36]
DA	MWCNT/PAMAM/ NiTsPc/GCE 0.1 M H2SO4	CV	2.5–240	0.54	-	-	[156]
DA	f-MWCNT/Nano-FeTSPc/GCE 0.1 M PBS, pH 7	SWV	20–51	0.35	-	-	[157]
DA	CoPc/Gr/GCE 0.2 M PBS, pH 4	SWV	3–75	-	-	-	[158]
DA	Nafion/CuTsPc/PANI/GCE 0.1 M H2SO4	FIA	0.01–1000	0.005	-	-	[159]
DA	MWCNT/NiTsPc/AGCE 0.1 M PBS, pH 4.0	SWV	0.02–1384	0.001	Human serum	98.4–100.7	[160]
DA	N-G/NiTsPc/GCE 0.1 M PBS, pH 7.4	Amp	0.1–200	0.1	-	-	[161]
DA	rGO/Mn-TPP/GCE 0.05 M PBS, pH 7	Amp	0.3–188.8	0.008	Pharmaceuticals Human urine	90.06–92.06 90.6–97.5	[162]
DA + AA + UA	MWCNTs/PdTAPc/GCE 0.25 M H2SO4	CV	DA: 2–16 AA: 3–24 UA: 5–40	DA: 0.6 AA: 1.0 UA: 1.5	Human urine	DA: 102–103.3 AA: 99–100.6 UA: 99.9–101.3	[163]
DA	CNP/FeTCAPc/GCE PBS (pH 7)	CA	0.05–0.50	0.016	-	-	[164]
DA	CNP/polyFeTBImPc/GCE PBS, pH 7	CA	0.05–0.50	0.01	Pharmaceuticals	100.32–102	[164]
DA	CoTSPc/Gr/GCE 0.1 M PBS, pH 7.3	DPSV	0.02–220	0.00087	-	-	[15]
DA	CoPc/GCE 0.1 M PBS, pH 7.4	DPV	0.223–8.513	0.0205	-	-	[165]
DA + UA	MnPc/GCE 0.1 M PBS, pH 7.4	DPV	DA: 0.029–6.239 UA: 1.127–10.977	DA: 0.0020 UA: 0.0146	-	-	[165]
DA + UA	ZnPc/GCE 0.1 M PBS, pH 7.4	DPV	DA: 0.021–7.441 UA: 0.014–8.577	DA: 0.0105 UA: 0.0305	-	-	[165]
DA + AA	MWCNT/CoTMBANAPc/GCE PBS, pH 7	DPV	DA: 7.5–67.5 AA: 7.5–67.5	DA: 0.33 AA: 6.66	-	-	[166]
DA + AA + UA	CoTGPc/GCE PBS, pH 7	DPV	DA: 2–12 AA: 2–12 UA: 2–12	DA: 1.2 AA: 0.5 UA: 0.5	Pharmaceuticals Urine	DA: 107.5 AA: 102 UA: 96.7–101.2	[58]
DA	Poly(CoTNBAPc)/GCE 0.1 M PBS, pH 7	Amp	0.1–1	0.02	Pharmaceuticals	102–104	[13]
DA + UA / DMZ	CoTfurNH2Pc/GCE PBS, pH 7	DPV	DA: 3–21 UA: 2–14 DMZ: 0.3–2.1	DA: 1 UA: 0.66 DMZ: 0.1	Milk powder Urine	DA: – UA: 99.8–101.2 DMZ: 97.5–101.0	[167]
DA + AA + UA + XO	Fe3C@NGCSs/GCE PBS, pH 6	DPV	DA: 1.2–120.8 AA: 54–5491 UA: 4.8–263 XA: 4.8–361	DA: 0.34 AA: 16.7 UA: 1.4 XA: 1.5	Serum	DA: 98.8 AA: 103.16 UA: 97.26 XA: 101.22	[168]
DA	rGO/ZnTPEBIPc/GCE PBS, pH 4	Amp	0.02–1	0.006	Pharmaceuticals	96.7–107.5	[169]
DA + AA + UA	AgNPs/Cu-MAPA/GCE 0.1 M PBS, pH 7.4	DPV	DA: 0.01–10 AA: 0.01–10 UA: 0.01–10	DA: 0.0007 AA: 0.0025 UA: 0.005	Articial urine	94–107	[170]
DA	NGQD/CoPc/GCE 0.1 M PBS	CA	100–1000	0.12	-	-	[171]
DA + UA	Gr/Car-ZnPc/GCE 0.1 M PBS, pH 7	DPV	DA: 2–16 UA: 20–160	DA: 0.079 UA: 0.33	Tap water	UA: 93.33–95	[172]
DA + UA	Gr/Car-CoPc/GCE 0.1 M PBS, pH 7	DPV	DA: 2–14 UA: 12–84	DA: 0.206 UA: 0.53	Tap water	-	[172]
DA	TACoPc/PANI/GCE 0.1 M PBS, pH 7	CA	20–200	0.064	Pharmaceuticals	103–103.6	[173]
DA + AA + UA	Poly-3N-CoPc/GCE PBS, pH 7.4	DPV	DA: 3.83–19 AA: 1.4–60 UA: 3.94–53	DA: 3.12 AA: 0.49 UA: 0.87	-	-	[174]
DA + AA + UA	Poly-3N-CuPc/GCE PBS, pH 7.4	DPV	DA: 3.71–19 AA: 3.69–53 UA: 3.55–53	DA: 2.53 AA: 0.52 UA: 1.31	-	-	[174]
DA + UA	CoPc/GQDs/GCE 0.001 M PBS, pH 7	DPV	DA: 2.91–16.2 UA: 10.76–3003	DA: 0.021 UA: 0.145	Human urine	DA: 91.3–104.5 UA: 91–95.2	[175]
DA + PAR	Poly-CoTAPc/ERGO/GCE 0.1 M PBS, pH 7.4	DPV	DA: 2–100 PA: 7–90	DA: 0.095 PA: 0.104	Synthetic urine	DA: 99.5–103 PA: 96–101	[176]
DA	Clicked-α-CoPc-flav3/GCE PBS, pH 6.3	SWV	2–14	0.31	Wastewater	97.1	[177]
DA	PPO/CoPc/CPE 0.05 M PBS, pH 7.4	Amp	Up to 1.8	7.5	Urine	-	[178]
DA + 5-HT	FePc/CPE FeTSPc/CPE FeTAPc/CPE TRIS, pH 7.4	OSWV	DA: 1–15 5-HT: 1–15	DA: 1 5-HT: 1	-	-	[47]
DA + AA	FeTSPc/CPE pH 7.4	CV	-	DA: 0.45 AA: 0.75	-	-	[179]
DA + AA	Nano-CoPc/CPE 0.025 M PBS, pH 7.4 + 4×-10−4 M CTAB	DPV	DA: 3–100 AA: 5–300	DA: 1 AA: 1.7	Pharmaceuticals	DA: 97.5–103.9 AA: 96.9–104.5	[180]
DA	MWCNT/FePc/CPE PBS, pH 7.4	DPV	5–25	0.205	Serum	-	[181]
DA	Si-Db/CuTsPc/CPE 0.1 M PBS, pH 4.5	CA	9.9–107.1	0.42	-	-	[182]
DA	(PAH/NiTsPc)5/ITOE 0.05 M H2SO4	CV	-	0.089	-	-	[183]
DA	(PAH/Chicha/PAH/NiTsPc)5/ITOE 0.05 M H2SO4	CV	0.025–3	0.105	-	-	[183]
DA	PAH/FePc/AgNP/ITOE 0.1 M KCl	CV	2–97	0.86	-	-	[184]
DA	DS01/NiTsPc/ITOE 0.05 M H2SO4	CV	0–19.6	1.665	-	-	[185]
DA	(PEI/Na+MMT/PEI/NiTsPc)10/ITOE 0.1 M PBS, pH 7.0	DPV	5–150	1	Human urine	94–111	[186]
DA + L-Dopa	FePc/ED/ITOE 0.1 M KCl	DPV	DA: 2–80 LD: 2–120	DA: 0.288 LD: 0.564	-	-	[18]
DA	CoTAPc-GO/Cu-Bi2WO6/ITOE BRB, pH 7.4	PEC sensor	0.05–5 5–250	0.0072	Pharmaceuticals	99.2–106.7	[187]
DA	ZnPc-P8BT-Pdots/ITOE	PEC sensor	0.0025–125	0.00169	-	-	[188]
DA	CoTCPhOPc/PEA/AuE PBS, pH 7.4	CV	5–100	1.32	-	-	[189]
DA	AmGQDs-CoTCPhOPc/IPA/AuE 0.1 M PBS, pH 7.4	DPV	1–50	0.20	Calf serum	101–106	[190]
DA	CoOCAPc/PEA/AuE 0.1 M PBS, pH 7.4	DPV	50	0.064	-	-	[191]
DA	FeOCAPc/PEA/AuE 0.1 M PBS, pH 7.4	DPV	1–50	0.25	Calf serum	96.9–105	[192]
DA	CoTBuPc/IL/SPGE 0.1 M H2SO4 + 0.01 M KCl, pH 1	CV	3.9–100	1.2	-	-	[193]
DA + AA	NiPc/CCE 0.5 M KCl, pH 5	DPV	DA: 40–1080 AA: 90–2110	DA: 0.26 AA: 0.45	Synthetic	DA: 107.25–108.25 AA: 101.25–108.75	[194]
DA	SiO2/C/CuPc disk electrode 0.08 M BRB, pH 6	Amp	10–140	0.6	Synthetic	100	[195]
DA	NiTsPc/ZnONPs/CNT/PGE 0.1 M KH2PO4, pH 3.4	DPV CA	0–15 0–7	0.007 0.024	Human serum	100.83–104	[196]

summarizes the electrochemical studies that have addressed the determination of DA using MPc-modified electrodes, as well as the performance characteristics of the developed methods. As can be seen, more than half of them have used a glassy carbon electrode (GCE) [13,15,36,38,58,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177] as a substrate for surface modification. However, carbon paste [47,178,179,180,181,182], indium tin oxide [18,183,184,185,186,187,188] and gold [189,190,191,192] electrodes also have unique properties that make them useful for DA detection.

One challenge in DA quantitative determination is its low concentration in biological samples, such as serum, urine and plasma, so the analytical methods developed must be sensitive. The synergistic effect of modifying electrode surfaces with MPcs and different types of nanomaterials has led to numerous composite sensors with different catalytic properties compared with unmodified electrodes. At the same time, the modified electrodes have significantly increased the peak current of DA redox reactions, allowing for lower detection limits and a greater linearity range.

The main challenge of all the investigations remains determining the DA in the presence of other biologically important molecules, such as AA and UA, which are compounds that coexist at high concentrations in biological matrices.

It was possible to determine DA and AA in the same sample due to the electrocatalytic oxidation of these compounds using CPEs modified with FeTSPc [179] and nanosized CoPc particles [180]. In the first case, the concentrations of DA and AA could be determined from the shift of the half-wave potential as a function of the concentration ratio of the two compounds. In the other case, the anodic peak potentials of DA and AA were separated with good sensitivity in the presence of CTAB. The simultaneous and sensitive electrochemical determinations of DA and AA were also possible with MWCNT-decorated CoPc-modified GCE [166] and with NiPc-modified ceramic matrices prepared by the sol–gel method [194]. The modified electrodes were stable, sensitive and reproducible for the micromolar detection of AA and DA.

GCEs were also used as substrates for modification to simultaneously determine DA and UA. Electrode surface properties were improved by electropolymerizing MPcs containing thiazole moieties (Mn, Zn) [165], drop-covering CoTfurNH₂Pc [167], and electropolymerizing or electrodepositing carbazole-linked ZnPc and CoPc [172].

There are also electrochemical methods that use MPc-modified electrodes and can determine DA, AA and UA simultaneously. PdTAPc immobilized on the GCE surface through self-assembled monolayer technique, as well as the larger surface area with an enriched active site of the MWCNTs, induced the faster electron transfer during the electrochemical sensing of AA. The MWCNT-dispersed PdTAPc-modified electrode was used for the simultaneous determination of DA, AA and UA in urine samples [163]. A simple and green method was used to fabricate CoPc embedded with ganciclovir units to modify the GCE surface. Using the modified electrode, AA was determined from commercial tablets, DA from dopamine hydrochloride injection and UA in human urine samples [58]. Chen et al. [168] described a pyrolytic method for the preparation of ultrafine Fe₃C nanoparticles incorporated into N-doped graphitic carbon nanosheets, with the hybrid nanocomposite being placed on a GCE. The modified electrode exhibited increased peak currents and enhanced peak separations for AA, DA, UA and XA oxidation due to its distinctive porous configuration and cooperative effects between N, Fe and carbon capsules. Studies of simultaneous determination were also performed using a Cu-MAPA polymer-modified electrode with silver nanoparticles for AA, DA and UA. The modified electrode was shown to have good electrochemical catalytic activity for the electrooxidation of the species, enhancing the analytical signal and reducing the overpotential compared with studies on bare electrodes [170]. Another study investigated the sensor performances of piperidine-substituted CoPc and CuPc for the detection of DA, AA and UA. Non-peripheral CoPc exhibited a good linear range and low detection limit for the electrochemical determination of AA. Meanwhile, non-peripheral CuPc demonstrated a high sensitivity and a low detection limit, particularly for AA and UA [174].

There is considerable interest in developing electrodes for the electrochemical determination of DA and other important molecules, particularly neurotransmitters. At conventional electrodes, DA exhibits oxidation peak potentials that are very close to those of EP, NP and 5-HT. This results in overlapping voltammetric responses, which makes it difficult to detect them electrochemically. Oni and Nyokong [47] used CPEs containing FePc and [FeTSPc]⁴⁻ to detect DA and 5-HT. They compared the catalytic activities of FePc and [FeTSPc]⁴⁻ with those of CoPc, NiPc [CoTSPc]⁴⁻ and [NiTSPc]⁴⁻ complexes. The results showed that the CoPc and NiPc complexes exhibited less catalytic behavior toward detecting 5-HT and DA than the FePc complexes. The simultaneous determination of 5-HT and DA on FeTSPc/CPE revealed that these species did not significantly interact with each other. Moreover, AA did not interfere with the detection of DA or 5-HT.

The versatility of FePc to form distinct supramolecular arrangements in thin films was used to adjust the electro-oxidation of catecholamines. The production of thin films of FePc on ITO electrodes was achieved using both Langmuir–Schaefer and electrodeposition techniques [18]. Electrodes modified by electrodeposition exhibited two distinct oxidation peaks in mixtures of DA and its amino acid precursor, L-Dopa. The separation of oxidation peaks can be attributed to different interactions with the surface of the electrodeposited FePc film. Moreover, the higher oxidation currents observed at the modified electrodes suggest that the FePc molecules are arranged in a manner that favors electron transfer from the analyte to the electrode [18].

4.2. Determination of Epinephrine and Norepinephrine

Both EP (adrenaline) and NP (noradrenaline) are catecholamines that function as neurotransmitters that signal between nerve cells, as well as hormones that travel through the bloodstream [197]. These compounds can impact various physiological functions, including the heart rate, brain muscle activity, blood sugar levels and blood vessel circulation. Although it increases both the heart rate and blood pressure, EP primarily affects the heart and muscles, while NP has a greater impact on blood vessels and blood pressure regulation [198]. They both play a critical role in the body’s “fight-or-flight” response, but they have distinct clinical significances. EP is used in emergency situations, like anaphylaxis, cardiac arrest and to treat asthma [199], while NP is used for conditions requiring sustained vasopressor support, particularly in septic shock and critical hypotension [200].

The EP and NP structures differ only by a methyl group on the amine side chain (Figure 3). Because of this, they have very similar electro-oxidation potentials, making it difficult to distinguish one from the other when they are present in a binary mixture. Consequently, few studies have examined the simultaneous determination of EP and NP. Several methods have been developed using unmodified electrodes and different voltammetric techniques. These include carbon fiber microelectrodes and fast-scan cyclic voltammetry [201], GCEs and DPV combined with chemometric methods [202], printed carbon electrodes and SWCV [203], and PGEs and SWCV [197]. However, some studies also used modified electrodes. Examples include a carbon nanotube paste electrode modified with 2-(4-oxo-3-phenyl-3,4-dihydro-quinazolinyl)-N-phenyl-hydrazine carbothioamide [204], a MWCNT-modified edge-plane pyrolytic graphite electrode [205] and a glassy carbon electrode modified with CTAB-assisted SnO₂ nanoparticles synthesized by microwave irradiation [206].

As shown in

Table 2. Electrochemical sensors based on phthalocyanines reported for EP and NP detection.

Analytes	Electrode and Medium	Detection Technique	Linear Range [×10−6 mol·L−1]	Detection Limit [×10−6 mol·L−1]	Sample	Recovery (%)	Ref.
EP	FePc/CPE 0.1 M ABS, pH 4	DPV	1–300	0.5	Pharmaceuticals	103.8	[207]
EP	Paraffin-MWCNT-CoPc/CPE 0.1 M PBS, pH 6	DPV	1.33–5.5	0.0156	Human urine	95.7–98.1	[208]
EP	MWCNT/Fe3O4/29H,31H-P/GCE PBS, pH 7.2	DPV	7.5–56	4.6	-	-	[57]
EP	MWCNT/ZnO/29H,31H-P/GCE PBS, pH 7.2	DPV	7.5–56	6.5	-	-	[57]
EP	CoTCPhOPc/PEA/AuE PBS, pH 7.4	CV	5–100	3.08	-	-	[189]
EP	AmGQDs-CoTCPhOPc/IPA/AuE 0.1 M PBS, pH 7.4	DPV	1–50	0.23	Calf serum	89–92	[190]
EP	CoOCAPc/PEA/AuE 0.1 M PBS, pH 7.4	DPV	5–50	0.22	-	-	[191]
EP	FeOCAPc/PEA/AuE 0.1 M PBS, pH 7.4	DPV	1–30	0.45	Calf serum	101–106	[192]
EP	CoTBuPc/IL/SPGE 0.1 M H2SO4 + 0.01 M KCl, pH 1	CV	0.29–100	1.13	-	-	[193]
NP	MWCNT/Fe3O4/29H,31H-P/GCE PBS, pH 7.2	DPV	7.5–48	2.2	-	-	[57]
NP	MWCNT/ZnO/29H,31H-P/GCE PBS, pH 7.2	DPV	7.5–48	1.7	-	-	[57]
NP	CoTCPhOPc/PEA/AuE PBS, pH 7.4	CV	5–100	2.11	-	-	[189]
NP	AmGQDs-CoTCPhOPc/IPA/AuE 0.1 M PBS, pH 7.4	DPV	1–50	0.45	Calf serum	85–102	[190]
NP	CoOCAPc/PEA/AuE 0.1 M PBS, pH 7.4	DPV	0.5–50	0.17	-	-	[191]
NP	FeOCAPc/PEA/AuE 0.1 M PBS, pH 7.4	DPV	1–50	0.34	Calf serum	93.5–100	[192]

, MPcs were not effective as working electrode surface modifiers for the simultaneous determination of EP and NP. However, studies investigated the electrochemical behavior and quantitative determination of the two transmitters separately. Some of these were performed in an attempt to detect them in the presence of dopamine [189,190,191,192,193].

In this context, the combination of MWCNTs, metal oxides (Fe₃O₄, ZnO), nanoparticles, and phthalocyanines employed for GCE surface modification resulted in enhanced EP and NP oxidation signals compared with using a bare GCE [57]. The catalysis of EP was favored on MWCNT/Fe₃O₄/29H,31H-P/GCE, while MWCNT/ZnO/29H,31H-P/GCE exhibited the highest sensitivity for NP.

A series of studies [189,191,192] examined the covalent immobilization of thin films of CoTCPhOPc, CoOAClPc and FeOAClPc on PEA-grafted gold electrodes. The presence of carboxylic acid terminal groups on the modified gold electrodes was confirmed by negatively and positively charged redox probes, [Fe(CN)₆]^3−/4− and [Ru(NH₃)₆]^2+/3+, respectively. Modified electrodes enabled the determinations of DA, EP and NP, as well as the screening of ascorbic acid [189,190,191,192,193,194,195].

It is known that carbon paste is a convenient matrix for incorporating chemical modifiers, so CPEs modified with FePc [208] and CoPc [209] have been proposed for the determination of EP in the presence of uric acid or ascorbic acid. Electrochemical investigations revealed the electrode’s efficient catalytic activity for the electro-oxidation of EP, improving the reversibility of the modified electrode response and decreasing its overpotential [208,209].

Miniaturized sensors were developed using CV to test screen-printed electrodes modified with ionic liquids for the detection of DA, EP and dobutamine in aqueous solutions. The ionic liquids played a dual role, acting both as a binder and a chemical modifier. Using CoTBuPc as an electron-conductive additive increased the electrochemical signals and lowered the detection limits [193].

4.3. Determination of Serotonin

Serotonin (5-HT, 5-hydroxytryptamine) is an inhibitory neurotransmitter that can also act as a neuromodulator, affecting appetite, learning, and mood and sleep regulation [209]. It also controls other processes, such as learning, memory, body temperature, sleep and appetite. Irregularities in its optimization in the nervous system can cause illnesses, such as liver regeneration, depression, anxiety, stress and psychosis [210]. Additionally, there is a connection between the decreased density of serotonergic receptors and aging and Alzheimer’s disease [211]. Disruptions in serotoninergic and noradrenergic systems are associated with some eating disorders, such as bulimia and anorexia nervosa [212].

As in the case of other neurotransmitters, problems encountered when determining 5-HT levels include the presence of other electroactive biomolecules and the low concentration of 5-HT in biological samples. Additionally, 5-HT oxidation products can adsorb onto the surface of conventional electrodes. A literature review [213] revealed that although several methods have been developed for the electrochemical determination of 5-HT, only a few address the use of MPcs for the surface modification of working electrodes (

Table 3. Electrochemical sensors based on phthalocyanines reported for 5-HT detection.

Analytes	Electrode and Medium	Detection Technique	Linear Range [×10−6 mol·L−1]	Detection Limit [×10−6 mol·L−1]	Sample	Recovery (%)	Ref.
5-HT	NiTSPc/Nafion/CFE 0.1 M PBS, pH 7.4	SWV	0.005–0.09	0.0038	Urine	92–93	[214]
5-HT	Tyr-CoPc/CPE 0.1 M PBS, pH 7	Amp	4–140	0.84	Walnuts	97–104	[215]
5-HT	G-FePc/GPE 0.1 M KCl + PBS, pH 3	DPV	1–1000	0.76	Urine	96.41	[216]
5-HT	GR-FePc/GPE 0.1 M KCl + PBS, pH 3	DPV	1–1000	0.734	Urine	97.07	[216]
5-HT	(PAH/NiTsPc/PAH/ CuNPs)/ITOE 0.1 M BRB, pH 2.2	CV	0.35–135	0.13	-	-	[217]

).

Among the studied metallic phthalocyanines, NiTsPc is the most promising metallic phthalocyanine for 5-HT electroanalysis. Compared with unsubstituted NiPc, the sulfonated groups of NiTsPc decrease the electronic density of the metal center and facilitate Ni oxidation [217]. Layer-by-layer films containing alternating layers of CuNPs and NiTsPc in indium-doped tin oxide electrodes were used to produce a composite material with high electrochemical stability and sensitivity for detecting 5-HT [217]. De Irazu et al.’s study [214] revealed that carbon fiber microelectrodes modified with NiTsPc and Nafion coatings exhibited electrochemical activity for 5-HT oxidation. The Nafion layer eliminates interference from ascorbic acid. Other catecholamines (EP and DA) are oxidized at different potentials and do not interfere with the 5-HT determination.

A large-scale study examined the ability of several dot microsensors based on carbon materials (graphite, graphene and carbon nanopowder) to detect 5-HT in urine samples [216]. The plain paste obtained from each of these materials and paraffin oil was modified with TiO₂, Pc, Fe(III)Pc and Fe(II)Pc. The authors demonstrated that G-Fe(II)Pc produced the most stable and sensitive results. However, the most consistent and lowest limits of detection for 5-HT were recorded using G-Fe(III)Pc- or GR-Fe(III)Pc-modified microsensors.

One fascinating topic in the field of electrochemical sensor construction is that of biosensors, in which enzymes are immobilized on or inside the supporting substrate and serve as recognition elements. A biosensor was fabricated for the detection and quantification of 5-HT using carbon nanopowder as the substrate, tyrosinase as the biocatalyst and CoPc as the electron mediator. Tyrosinase was immobilized on the carbon nanopowder using the drop-and-dry method, followed by cross-linking with glutaraldehyde [215]. The determination of 5-HT in walnut samples was performed using amperometry, yielding good results.

5. Conclusions and Future Prospects

This review surveys several approaches adopted for the selective quantification of monoamine neurotransmitters using specific MPcs. We describe several possible applications of MPcs-modified electrodes, emphasizing the remarkable properties of MPcs and carbon nanomaterials that create a host of application possibilities in sensor design. The interest in this field arises from the versatility of MPcs and carbon materials and the ability to generate novel properties and applications. Future efforts will need to be directed toward increasing the selectivity and sensitivity of MPc-modified electrodes by preventing the non-specific adsorption of molecules onto the electrode surface. Further improvements may be expected by expanding the range of modifying molecules that can be attached to the electrode surface; so far, enzymes and nucleic acids have been mostly employed for this purpose. However, the development of a sensing platform with high sensitivity and selectivity is challenging, and it has been found to be a bottleneck step in the analysis of neurotransmitters.