1. Introduction
The pervasive conveniences of modern life are a testament to the remarkable advancements in science and technology. We consistently rely on various devices such as computers, phones, refrigerators, air conditioners, and smoke detectors for seamless interaction with the physical world [
1]. Most of these devices would not operate without sensors, which act as our electronic eyes and ears [
2]. Electronic devices detect changes in physical or chemical properties, such as pressure, temperature, humidity, motion, and light, by converting them into electrical signals for processing and analysis [
3]. Ideal sensors are imperative components of various measurement systems, possessing specific characteristics such as a wide operational range, minimal drift, ease of calibration, high sensitivity, and selectivity [
4]. Sensor technology has gained importance in numerous fields, including environmental and food quality monitoring, medical diagnosis and healthcare, automotive and industrial manufacturing, space exploration, and national security [
5]. The development of sensors has led to their use in diverse fields, and one particularly exciting area is biosensors. The term ““biosensor”” was coined by Cammann, and its definition was established by IUPAC [
6]. Biosensors detect biological processes, diagnose diseases, identify environmental contaminants, and aid in drug discovery. Biosensors function by detecting biomolecules such as proteins, DNA, RNA, enzymes, and hormones, providing valuable insights into health conditions, disease progression, and environmental security [
7]. Promptly and accurately identifying biomarkers plays a crucial role in advancing early disease diagnosis and personalized medicine [
8]. Biosensor development is continuously advancing with a focus on designing highly sensitive and selective devices for biomolecular detection. However, biosensors face many challenges. Their accuracy and specificity are hampered by complex biological samples and environmental interferences. Device performance in terms of temperature and humidity requires careful control [
9]. Additionally, some biosensors have a limited lifespan owing to degradation and require regular calibration for sustained accuracy. Furthermore, the development and maintenance of biosensors is expensive because specialized materials and expertise are required [
9].
Figure 1 shows a schematic diagram of the biosensor device used in this study. The biosensor operates via a series of steps involving the interaction of a target analyte with a bioreceptor, leading to the generation of a measurable signal.
Ongoing research is focused on improving biosensor sensitivity, selectivity, and stability, paving the way for wider application in the future. From this perspective, the fabrication of biosensors has gained significant attention for advancement in device functionality and practical applications [
11]. The quest for precise and efficient biosensor design and fabrication technique is paramount for unveiling the potential of “smart” biosensor systems [
12]. Various biosensor technologies enable biomolecule detection, each with its own operating principle. Optical biosensors use light interactions with biomolecules to measure them, piezoelectric biosensors track mass changes upon biomolecule binding, surface plasmon resonance (SPR) biosensors detect refractive index changes caused by complex biomolecules, and electrochemical biosensors measure the electrical signals produced during biomolecule binding at the electrodes [
13]. Electrochemical biosensors are promising because of their sensitivity, affordability, portability, biocompatibility, simplicity, and fast response [
14]. A biochemical receptor captures the response of a biological process and transmits it to a transducer [
14]. An ideal biosensor is independent of temperature and pH, recyclable, and specific [
15]. Electrochemical biosensors consist of three electrodes: reference electrode (RE), counter electrode (CE), and working electrode (WE) [
16]. Electrical signal detection involves electrons that are generated or consumed, and it is categorized as potentiometric, cyclic voltammetric (CV), chronoamperometric (CA), differential pulse voltammetry (DPV), impedimetric, and linear sweep voltammetry (LSV) [
17]. The sensing mechanism includes the interaction between the analyte and the pinhole surface of the WE to induce a redox reaction. To decrease the overpotential and fouling effect and improve the response, virgin electrodes have been modified with appropriate redox materials or stimulant [
17]. To optimize the interaction, an electrode surface has been fabricated with various conducting nano and rare noble materials (Au, Pt, etc.) to facilitate faster electron movement in biosensors [
18]. Different redox-active materials such as NPs, metal oxides, carbonaceous materials, polymers, macrocycles, and organic compounds have been employed on the electrode surface because of their large surface area, enhanced conductivity, optical properties, and biocompatibility [
18]. However, long-term stability, reproducibility, and cost-effectiveness of biosensors remain challenging issues [
19]. In response, various hybrid materials such as graphene oxide (GO)–NPs hybrids [
20], polymer–enzyme hybrids [
21], carbon nanotube (CNT)–quantum dot hybrids [
22], metal–organic framework hybrids [
23], and nanostructured composite hybrids [
24] have been extensively designed for the development of novel materials with exceptional properties.
Organic-based macrocycles (phthalocyanine (Pc)/porphyrin) are considered reliable materials for signal amplification in electrochemical sensing owing to the simplicity of their synthesis and the ability to fine-tune their electronic properties through the substitution of axial/peripheral groups [
25]. N4 macrocycles Pc are interesting because of their unique electronic structure, substantial surface area, distinctive atomic structure, and properties that can be engineered by substituting various functional groups or metal ions [
26]. Pc consists of a tetrapyrrole core similar to that of porphyrins in natural systems. This structural resemblance to the heme group in hemoglobin and chlorophyll in photosynthetic organisms is one of the reasons for their bioinspired appeal [
26]. The central metal ion in Pc plays a vital role in tuning the catalytic properties. Transition metals, such as Fe, Co, Mn, and Ni, are commonly used as metal centers in Pc [
26]. Furthermore, the rigid and planar structure of Pcs provides stability and promotes efficient electron and ion transport during catalytic processes [
27]. Additionally, their notable features include high selectivity and a specific size for miniaturized binding geometries, which reduce interference and ensure more accurate data [
28]. Furthermore, it offers improved detection limits, enhanced sensitivity, and signal amplification for targeted molecules at minute concentrations, making it a versatile material for biosensing applications [
28]. However, the design and fabrication of hybrid materials ensures high stability in harsh environments, and their smooth integration with smart devices underscores the utilization of macrocyclic complexes in biosensor applications.
In recent years, the design and fabrication of biosensors using hybrid materials containing Pc with metal oxide or carbon composites has increased, owing to the large scope of functional operations [
29]. Hybrid materials provide unique exploitation owing to the collaborative attributes of each component. Pcs contribute to tunable biocompatibility, leading to improved sensor response and easier processing during fabrication [
30]. Carbon nanomaterials (CNs) substantially enhance the electrical conductivity and surface area of biosensors, allowing improved detection efficiency [
31]. Furthermore, the incorporation of metal nanoparticles (MNPs) further enhances the sensing performance through their catalytic activity, signal amplification, and unique optical properties [
31]. Hence, the deployment of Pc–metal oxide or polymer–metal–carbon (PMC) hybrids has led to the development of biosensors with superior sensitivity, enabling the detection of analytes at lower concentrations [
32]. Additionally, PMC hybrids offer improved selectivity, meaning that they can distinguish between the target molecule and potential interferences. The use of these hybrid materials paves the way for the miniaturization of biosensors, making them smaller, more portable, and potentially more suitable for various biomedical applications. Despite these positive observations, ensuring long-term stability, biocompatibility, reproducibility, minimal potential toxicity of NPs, and developing cost-effective production methods, as well as fabricating PMC-based hybrid biosensors, remain paramount challenges [
30,
31,
32]. In this review, we briefly introduce hybrid materials. The synthesis strategies, properties, and sensing mechanisms of the hybrid materials are also incorporated, and their potential for the detection of analytes such as biomarkers, environmental pollutants, and food contaminants is explained. Furthermore, we explore recent advancements in the development and application of PMC-based hybrid materials for biosensors, particularly the integration of sensor systems, and their translation into practical applications. Finally, the challenges and future directions of PMC-based hybrids for biosensors are discussed.
3. Pc-Based Hybrid Composites
In recent years, the integration of an organic hybrid composite of Pc with metal oxides or CNs in biosensors has been promising owing to the unique combination of properties and synergistic framework. Pc provides tunable biocompatibility and enhances sensor responsiveness and processability, whereas CNs (CNTs, graphene, and GO) contribute to electrical conductivity and a large surface area [
99]. MNPs exhibit catalytic activity, signal amplification, and unique optical properties that further enhance the sensing competence of hybrid materials [
100]. The integration of PMC-based hybrids reinforces the performance of biosensors with improved sensitivity, detection limit, selectivity, multifunctionality, and miniaturization [
101]. The improved biosensing performance of hybrid composites is explained below.
a) Combining MPc with carbon materials in a hybrid composite overcomes the MPc aggregation issue and enhances biosensor sensitivity [
102].
b) An 18 п-electron system in MPc combined with carbon materials results in the optimization of hybrid composites with high porosity, surface area, and conductivity. This contributes to improved specificity in detecting biomolecules and enhances selective detection [
103].
c) The integration of carbon materials into a hybrid composite not only enhances electrical conductivity and electron mobility, but also facilitates efficient electron transfer, ensuring a rapid biosensor response [
104,
105].
d) Carbon materials in hybrid composites solve the stacking arrangement issue in MPc and improve dispersion, diffusion, and adsorption to overcome issues related to long-term stability [
106].
e) The immobilization of the hybrid composite on the electrode surface in a hierarchical structure leads to strong affinity and stability, thus contributing to an efficient biosensing process [
107].
Figure 9 illustrates the step-by-step preparation process of a laccase-based biosensor on a screen-printed electrode (SPE) modified with cobalt Pc-modified carbon nanofibers (CoPc-CNFs). The fabrication procedure involves several key steps. Carbon nanofibers are initially treated to enhance their surface properties and facilitate the binding of the cobalt Pc molecules. Cobalt Pc, known for its catalytic properties and stability, is then immobilized onto the surface of the carbon nanofibers through a suitable deposition method, such as drop-casting or electrodeposition. An SPE is utilized as the substrate for the biosensor owing to its ease of fabrication, portability, and compatibility with different detection techniques. The SPE is fabricated by printing a layer of conductive ink onto a substrate and pasting CoPc-CNFs onto the WE surface area. The laccase enzyme has ability to catalyze the oxidation of phenolic compounds immobilized on the CoPc-CNFs-modified electrode surface. The immobilization process involves incubating the electrode in a laccase-containing solution under controlled conditions, allowing the enzyme to adsorb or bind to the electrode surface. The enzymatic oxidation mechanism of protocatechuic acid (PCA) in the presence of laccases is shown in
Figure 9(b). Laccase catalyzes PCA oxidation by transferring electrons from the substrate to molecular O
2, resulting in the formation of reactive intermediates that lead to the formation of quinone products. In the oxidation process, electrons are transferred from PCA to the laccase enzyme, which undergoes a redox cycle involving the active sites of copper ions (Cu
2+ and Cu
3+), facilitating the oxidation reaction. This enzymatic oxidation of PCA leads to the formation of a quinone product via the release of protons and electrons.
Figure 9 provides valuable insights into the fabrication and enzymatic oxidation mechanism of a laccase-based biosensor for the detection of phenolic compounds such as PCA. The integration of CoPc-modified CNFs for laccase immobilization enhances catalytic activity and sensitivity in biosensor applications.
Additionally, the incorporation of carbon materials like graphene or nanotubes disrupts π-π stacking in Pcs which hinders the accessibility of active sites for biomolecule interaction. The improved dispersion maximizes the surface area for biomolecule adsorption during sensor fabrication. Furthermore, the 18 π-electron system of MPcs interacts with the carbon network to optimize the electronic structure of the composite. This enhanced π-conjugation facilitates efficient electron transfer and improves interfacial charge transfer between the immobilized biomolecule and the composite, leading to superior detection level of Hydrazine (Hz). Additionally, the excellent conductivity of carbon materials significantly increases the overall conductivity of the composite, resulting in a fast response and increased sensitivity during fabrication. Moreover, the optimized structure during fabrication maximizes the surface area for biomolecule interaction, which promotes strong binding with target biomolecules and facilitates efficient mass transport, thereby boosting the sensitivity and response time of the biosensor [
109].
Hybrid composites of rGO have gained attention for biosensing applications because of their unique properties. rGO provides high conductivity and a large surface area, facilitating efficient electron transfer and increasing biomolecule interaction. This translates into potentially faster and more sensitive biosensors, aiding in the development of high-performance biosensors with modified electrode surfaces that exhibit minimal interfacial resistance, exceptional stability, and efficient electron transfer between the electrolyte and electrode. Recently, Jilani et al. [
110] explored the potential of carbonaceous and metal phthalocyanine (MPc) hybrid composites for nitrite-sensing applications. As shown in
Figure 10, they synthesized a novel composite material, cobalt (II) tetramethylquinoline oxy bridged Pc (CoTM-QOPc) and used it for nitrite sensing. The sensor exhibited a linear detection range of 0.3 to 120 μmol/L, using CV, and 0.2 to 170 μmol/L, using CA. The detection limit achieved was 0.1 μmol/L for CV and 0.06 μmol/L for CA, with good sensitivity of 0.765 μA μM⁻¹ cm⁻² (CV) and 1.204 μA μM⁻¹ cm⁻² (CA). Recognizing the potential for further improvements, they strategically incorporated CNP into the CoTM-QOPc matrix. This carbonaceous and MPc hybrid composite (CoTM-QOPc/CNP) offered an enhanced electrocatalytic response for nitrite oxidation compared to the electrode modified with Pc alone. The linear detection range for nitrite sensing using the composite electrode was even broader, spanning from 0.2 to 200 μmol/L (CV), 0.2 to 225 μmol/L (DPV), and 0.1 to 350 μmol/L (CA). The detection limit also improved remarkably, reaching 0.06 μmol/L for both CV and DPV, and 0.033 μmol/L for CA. The sensitivity of the CoTM-QOPc/CNP composite electrode was also superior, with values of 2.298 μA μM⁻¹ cm⁻² (CV), 1.031 μA μM⁻¹ cm⁻² (DPV), and 1.237 μA μM⁻¹ cm⁻² (CA). This highlights the remarkable utilization of carbonaceous materials in Pc-based sensors. The CoTM-QOPc/CNP composite electrode demonstrated not only enhanced sensitivity and detection limits for nitrite but also impressive selectivity, even in the presence of interfering ions such as AA, carbonate, urea, phosphate, and glucose. This study paves the way for the development of highly selective and sensitive biosensors for real-world applications. Similarly, Shambulinga et al. [
111] designed an oxy-bridge cobalt Pc polymer (poly(TazoCoPc)) to enhance the conjugation effect for nitrite detection. Furthermore, poly(TazoCoPc) doped with CNP was utilized for electrochemical voltammetric and amperometric nitrite sensors. The poly(TazoCoPc)/CNP composite demonstrated superior electrocatalytic activity for nitrite oxidation compared to pure poly(TazoCoPc). The amperometric sensor showed excellent performance in detection of nitrite concentrations ranging from 20 nM to 1 μM with a detection limit of 6 nM and a sensitivity of 0.137 mA/μM. The modified electrode exhibited high selectivity with no interference from ions such as Mg
2+, SO
42−, K
+, CO
32−, and NO
3−. Similarly, Manjunath et al. [
112] designed a cobalt (II) tetra-β-[N(2-(1,3-benzothiazole)) carboxamide] Pc (CoTBTCAPc) for detection of 4-aminophenol (AP) (
Figure 11). The GCE/CoTBTCAPc electrode displayed poor charge transfer, whereas the composite electrode with CNP (GCE/CNP-CoTBTCAPc) showed improved charge transfer. Both electrodes exhibited reduced overpotential and increased oxidation peak current. For 4-AP detection in phosphate buffer, they exhibited linear responses with detection limits of 13 nM (GCE/CoTBTCAPc) and 11 nM (GCE/CNP-CoTBTCAPc). DPV showed sensitivities of 0.0328 and 0.4179 μA nM
−1 cm
−2, while amperometry showed sensitivities of 0.4008 and 0.8887 μA nM
−1 cm
−2, with LODs of 40 and 30 nM. The GCE/CNP-CoTBTCAPc electrode was selective for 4-AP in the presence of interferents, making it suitable for the analysis of real samples, such as 4-AP in paracetamol tablets.
CNs, particularly MWCNTs, have immense potential as next-generation biosensors. In Pc-based composites, MWCNTs improve conductivity and enhance electron transfer with faster response times and potentially higher sensitivity. Additionally, the MWCNTs increase the effective surface area by providing more space for biomolecule interactions and potentially more binding sites. These CNs contribute to the overall stability of the composite by offering mechanical support, leading to a more robust and long-lasting biosensing activity. Hence, the synergistic effects of incorporating MWCNTs into Pc composites provide great possibilities for the development of superior biosensors with enhanced sensitivity, stability, and performance [
113,
114]. The electrocatalytic performance of a CoTEIndCAPc/MWCNTs/GCE electrode for Cd²⁺ and Pb²⁺ detection is illustrated in
Figure 12. The electrode was designed and utilized for CV, DPV, and CA measurements of electrochemical activity. The electrode showed excellent sensitivity with low detection limits of 10 nmol L⁻¹ for Cd²⁺ and 9 nmol L⁻¹ for Pb²⁺, and high reproducibility, highlighting its potential for biological applications [
114]. Recently, Mounesh et al. [
115] presented an intriguing approach for biosensing applications using carbonaceous and MPc hybrid composites. They synthesized a novel tetra-8-[(E)-(4-methoxybenzylidene) amino] naphthalene-1-amine cobalt (II) Pc (CoTMBANAPc) through an amide bridge linkage, using cobalt (II) tetracarboxylic acid Pc (CoTCAPc) as the initial material. This synthesized macromolecule displayed excellent solubility in aprotic organic solvents, providing valuable insights into the composition and structure of the material. The GCE/MWCNT-CoTMBANAPc electrode exhibited remarkable promise for the simultaneous detection of AA and DA using CV, DPV, and CA techniques. The detection performance within the linear response of the concentration range of 7.5 to 70 nM for both AA and DA. The composite exhibited a lower detection limit of 6.6 μM for AA and 0.33 nM for DA. Furthermore, the GCE/MWCNT-CoTMBANAPc electrode displayed excellent stability, sensitivity, and reproducibility within the micromolar range. However, exploring the selectivity of these sensors in complex biological matrices where additional interfering species are present is necessary. Keshavanand Prabhu et al. [
116] developed FeTBImPc, modified it with CNP, and immobilized it on a GCE for the detection of DA. The composite electrodes (GCE/CNP-FeTCAPc and GCE/CNP-polyFeTBImPc) showed excellent electrocatalytic activity toward DA oxidation, with lower detection limits of 14 nM. The GCE/CNP-polyFeTBImPc sensor exhibited superior performance with a high sensitivity of 67.2039 mA nM⁻¹ cm⁻². However, carbonaceous and metal–Pc hybrid composites face many challenges. The aggregation of Pc molecules limits the available surface area for biomolecule interaction and hinders sensor performance. The long-term stability of the composites under real-world conditions requires further investigation.
Figure 13 provides a comprehensive overview of the applications of hybrid Pc composites as biosensors. The cobalt (II) Pc-modified GCE exhibited enhanced performance for the detection of AA and DA.
Table 4 highlights the versatility of various hybrid Pc composites in terms of their sensitivity and selectivity [
109,
110,
111,
112,
113,
114,
115,
116,
117,
118].
4. Fabrication of Hybrid Pc in Three-Electrode System
The fabrication of hybrid Pc materials for electrochemical applications using a three-electrode system involves several advanced techniques aimed at significantly enhancing the conductivity, stability, and catalytic efficiency of the WE. In a typical three-electrode setup, the WE is where the electrochemical reaction of interest occurs, the CE completes the electrical circuit, and the RE maintains a stable potential, allowing for precise measurement of the potential and current of the WE. Thin film deposition techniques, such as chemical vapor deposition (CVD) and sputtering, are essential for achieving precise control over film thickness, consistency, and uniformity. In CVD, volatile precursors decay and react on the substrate, forming a thin, uniform film of Pc, which is crucial for the formation of deformity-free layers and effective electron transfer. Faltering involves the physical injection of material from an objective through high-energy ion bombardment. The material is then deposited onto the WE surface, providing a controlled and uniform coating that is fundamental for steady electrochemical performance. Electrochemical deposition further customizes the WE by integrating different metal ions into the electrode surface, thus fitting the catalytic properties of Pcs. This technique involves immersing the WE in an electrolyte containing metal ions and applying a voltage, which reduces the metal ions and deposits them on the electrode surface. This technique is particularly beneficial for framing nanostructured surfaces that significantly enhance catalytic activity and stability. Furthermore, self-gathering procedures, including the development of self-assembled monolayers (SAMs), are utilized to create coordinated molecular structures on the WE. SAMs involve immersing the WE in a solution containing particles with a functional group that binds strongly to the electrode material. These molecules spontaneously form a monolayer, providing a coordinated design that improves charge energy kinetics and sensor responsiveness by ensuring that the Pc molecules are optimally oriented for electron transfer.
Hybridization with nanomaterials, such as graphene, CNTs, rGO, and MNPs, is achieved through techniques such as solution-phase mixing, in situ development, or layer-by-layer assembly. This integration significantly expands the surface area and conductivity of the WE, leading to improved electron transfer rates and electrochemical performance. The collaboration between Pcs and nanomaterials results in composite materials with superior electron transfer rates and enhanced strengths under electrochemical conditions. Advanced lithographic methods, including photolithography and electron beam lithography, are used to design the WE with high accuracy, facilitating the formation of complex and miniaturized electrode designs. Photolithography uses light-sensitive photoresists to achieve micron-level accuracy, whereas electron beam lithography offers considerably greater resolution for nanoscale fabrication. These techniques ensure the optimization of the cathode surface for high electrochemical activity, making hybrid Pc materials ideal for various applications. Using these fabrication techniques, researchers can optimize the WE interface in a three-electrode system framework, thereby significantly enhancing the performance and reliability of electrochemical biosensors.