From Natural Pigments to Bioinspired Sensors: The Emerging Role of Melanins

Abstract

The growing demand for sustainable, biocompatible, and multifunctional sensing materials has intensified interest in melanin and its derivatives, including melanin-inspired polymers and composites. Melanin is a naturally occurring biopolymer whose intricate structure and diverse chemical composition give rise to a remarkable combination of optical, electrical, and chemical properties. Key physicochemical characteristics, such as broadband optical absorption, hydration-dependent conductivity, redox activity, and metal ion coordination, are closely linked to melanin’s signal transduction capabilities and underpin its relevance in sensing applications. Recent advances in melanin-based sensing technologies encompass pH, humidity, chemical, biological, and optical platforms, with particular emphasis on hybrid systems incorporating graphene, silicon, or nanomaterials, and printable or wearable device architectures. These developments have enabled enhanced performance and broadened potential application fields. However, persistent challenges, including intrinsic heterogeneity, limited selectivity, relatively low electrical conductivity, and poor long-term operational stability, still limit practical implementation. Emerging molecular engineering and advanced fabrication strategies are being developed to address these limitations. Together, these findings position melanin as a versatile, eco-compatible, and functionally rich material, with a significant potential to underpin the next generation of sustainable sensing technologies.

1. Introduction

Sensors are devices capable of transducing a physical, chemical, or biological stimulus into a measurable signal and serve as fundamental tools across a wide range of fields, including environmental monitoring, health diagnostics, food safety, and wearable electronics [1,2]. As the demand grows for sensors that are more sensitive, selective, biocompatible, flexible, sustainable, and capable of multifunctional detection, the design of materials with novel and advantageous combinations of properties becomes crucial [3,4]. A particularly promising class of bio-inspired materials comprises melanin and its derivatives, including melanin-inspired polymers or composites. Melanin-based sensors have attracted increasing attention in recent years [5,6,7,8] because of melanin’s unique optical, chemical, electronic, and structural properties, as well as its biocompatibility and potential for sustainable sourcing.

This growing interest is also reflected in the temporal evolution of the scientific literature, although a robust statistical or trend-based analysis remains challenging due to the relatively recent emergence and interdisciplinary scope of research specifically devoted to melanin-based sensors. Before the late 2010s, studies explicitly integrating melanin into sensor technologies were limited and largely centered on pigment characterization, melanometric measurements, or analytical quantification rather than on fully developed functional sensing platforms [9,10,11,12,13,14]. In the period that followed, research activity remained relatively modest and niche, with publications primarily reporting early proof-of-concept electrochemical or optical sensing approaches that leveraged the adhesive, redox-active, and metal-chelating properties of polydopamine [15,16,17,18]. From the early 2020s onward, a noticeable expansion of the field can be observed, marked by the emergence of dedicated review articles and a diversification of application areas, including environmental monitoring, food safety, biosensing, and wearable technologies. In more recent years, publication activity has increased substantially and appears to be sustained, driven by advances in melanin-based electrochemical interfaces, optical probes, molecularly imprinted sensors, and hybrid material systems [19,20,21,22].

Overall, melanin provides a multifunctional platform for developing a new generation of sustainable and high-performance sensors [19,23]. Ongoing research on its synthesis, characterization, and integration into diverse device architectures underscores its substantial potential to transform applications ranging from biomedical diagnostics and wearable electronics to environmental monitoring. The originality and novelty of this work arise from the systematized, updated, and practice-oriented presentation of a large number of selected works, with emphasis on recent findings and pivotal studies, regarding the potential applications of melanin-based materials in sensing.

2. Melanins: Types, Structure, and Occurrence

Melanin is a biopolymer present across all biological kingdoms, whose molecular structure remains only partially elucidated [24,25]. It serves different functions, including radiation protection, pigmentary and structural coloration, thermoregulation, and radical scavenging [23,24,26,27].

Melanin is a generic term, indicating a family of biologically significant pigments that can be classified, based on their chemical precursor and structure, into five main categories (summarized in Figure 1 and Figure 2 and

Table 1. Main classes of melanin polymers.

Melanin	Key Precursor	Main Functional Groups	Color
Eumelanin	L-DOPA	hydroxyl carboxyl indole	brownish-black
Pheomelanin	cysteinyl DOPA	hydroxyl carboxyl indole thiazole	reddish-brown
Neuromelanin	dopamine & cysteine	hydroxyl carboxyl indole	dark brown
Allomelanin	1,8 DHN	hydroxyl	brownish-black
Pyomelanin	homogentisic acid	hydroxyl carboxyl	reddish-brown

): eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin [24,27]. The chemical composition of these biopolymers includes the presence of diverse functional groups, which enable melanins to interact with various organic compounds, both through hydrogen bonding, and π-π stacking, and contribute to their varied coloration and functional properties [23,27,28,29,30].

Among these functionalities, the most abundant is the hydroxyl group—often occurring in its oxidized (quinonic) form—which characterizes all five classes of melanins, followed in prevalence by the carboxyl group [23,24]. In addition, nitrogen-containing moieties, such as indoles and amines, are typically present, except for allomelanin and pyomelanin [23,24].

Eumelanin, deriving from the oxidative polymerization of L-DOPA (L-3,4-dihydroxyphenylphenylalanine), itself a product of tyrosine metabolic pathway, is brownish-black and it is mostly present in animals, in mammalian skin, eyes, and hair [23,24,31,32]. Pheomelanin, whose key structural unit is cysteinyl-DOPA, is instead characterized by a yellow-reddish-brown color, typical of red hair and freckles [23,24]. It differs from eumelanin due to the presence of sulfur-containing units (i.e., benzothiazines and benzothiazoles) [24,29]; its biosynthesis indeed starts from the polymerization of 5-S-cysteinyl-dopa, itself deriving from dopaquinone in the presence of cysteine [24,28,30]. Neuromelanin contains both pheomelanin and eumelanin, and, as the name suggests, this pigment is found in the brain, but its function is still unclear [23,33,34,35]. Its formation involves the oxidative polymerization of dopamine and cysteine [23,33,34,35].

Allomelanin and pyomelanin are produced by fungi and bacteria. The latter derives from homogentisic acid, while 1,8-dihydroxynaphthalene (DHN) is the main precursor of the former [23,36,37,38,39]. Auricularia auricola is a notable example of a fungus able to synthesize great amounts of melanin, characterized by various biological activities, therefore making it interesting for industrial applications [37].

On a larger scale, melanin monomers assemble themselves into stacked oligomers, mainly through π–π interactions and hydrogen bonding. These subsequently cluster together to form larger particles, usually in the range between 100 and 200 nm, with a spacing between the stacked layers of about 0.34 nm [23]. This hierarchical organization plays a key role in determining melanin’s electronic and optoelectronic behavior [38].

3. Key Physicochemical and Electrical Properties of Melanin Polymers

The ubiquitous distribution of melanin highlights its biological importance [24,30], while its diverse physicochemical properties make it valuable for a wide range of applications, including biomedicine, bioelectronics, cosmeceuticals, and environmental bioremediation [23,28,32].

Its versatility is intrinsically linked to its unique physicochemical features (summarized in

Table 2. Summary of physicochemical and electrical properties of melanin polymers.

Property	Main Related Features	Main Related Applications
Broadband optical absorption	photoprotection abilities photothermal and photocatalytic activities	bioelectronic materials [21,40]. photothermal tumor eradication [41] organic synthesis [42]
Charge transport	tunable electronic and protonic conductivity	sustainable electronics [43,44,45,46] humidity sensing [19]
Metal chelation & redox activity	protective role against oxidative stress adsorption capacities for heavy metals	electrochemical energy storage [47] heavy metal removal [48,49]
Biocompatibility & biodegradability	easy removal reduced safety concerns	bioengineering [50,51] specific drug delivery [52,53,54,55]

), arising from its complex, highly variable molecular structure, which depends on its origin, type, and specific precursors [23,38].

3.1. Optical Properties

Melanin exhibits a broadband optical absorption extending across the ultraviolet (UV), visible, and near-infrared (NIR) regions [56,57]. This feature is due to its complex and heterogeneous polymeric structure; the polymerization of various indole and indole-quinone moieties, as well as their subsequent supramolecular assembly via hydrogen bonding, π−π interaction, or cation−π interaction, results in complex and diversified spectra, dependent on the structure of the monomeric species involved [38,39,56,57,58].

This property underpins one of melanin’s primary biological roles, photoprotection, by enabling the efficient dissipation of high-energy radiation and preventing subsequent photochemical damage [21,58].

Moreover, it has also motivated a wide array of innovative technological applications. Recent studies demonstrated that melanin–silicon hybrid junctions can serve as effective broadband photodetectors [21], while melanin nanoparticles, integrated with silk protein, could be exploited for the design of photoresponsive, self-healable optoelectronic skins [40], highlighting their potential in bio-integrated applications. Functionalized melanin nanoplates instead demonstrated their efficiency in photoacoustic imaging and photothermal tumor eradication [41]. Recently, several synthetic melanins have also been employed as heterogeneous photocatalysts, mediating the visible-light-driven reductive dehalogenation of a series of α-halogen carbonyl derivatives [42].

3.2. Charge Transport Properties

Melanin is also characterized by unique charge transport properties, exhibiting indeed both electronic and protonic conductivity [43].

Due to this remarkable feature, these biopolymers can be categorized as mixed ionic–electronic conductors (MIECs) and therefore be considered highly valuable materials in sustainable electronics applications, e.g., bioelectronics, neuromorphic computing, soft robotics, or energy storage [43,44,59], in addition to their possible employment as electrochemical sensors.

Recently, conditions determining the prevalence of one conductivity mechanism over the other have been widely investigated [43,44,45,46], and it emerged that the hydration state plays a major role in this sense: when wet or humid, proton conduction is facilitated by the presence of water molecules, whereas in drier conditions, electronic conduction may dominate [43,46].

This high sensitivity to moisture and changes in hydration also makes melanin considerable for humidity sensors and related applications [19].

3.3. Metal Chelation and Redox Activity

Melanins also exhibit significant metal-ion chelating activity due to the presence of the above-mentioned functional groups in their structure [48,60]. This ability to form stable complexes is crucial as transition metal ions, particularly iron and copper, can catalyze the formation of highly reactive free radicals through Fenton-type reactions, thereby exacerbating oxidative stress [61]. By binding them, melanin effectively inactivates their pro-oxidant potential, contributing to its overall antioxidant defense mechanism [60].

Moreover, melanin’s excellent radical scavenging ability—arising from its significant redox activity—is well-documented. Acting as a natural antioxidant, this biopolymer plays a protective role against oxidative stress—protecting cells and tissues from harmful reactive oxygen species (ROS) and free radicals—and environmental damage [62,63,64].

The mechanism of free radical scavenging often involves the donation of hydrogen atoms or electrons from the antioxidant to the free radical, thereby converting the radical into a more stable, non-radical species. Spectroelectrochemical reverse engineering has indeed demonstrated its ability to exchange electrons [64].

Both characteristics have been exploited in different applications. For example, eumelanin electrodes showed their potential in electrochemical energy storage due to their redox behavior [47]. Conversely, melanin’s chelating ability has been exploited in environmental applications, such as the removal of heavy metals from aqueous solutions. Biosynthesized melanin, derived from the marine bacterium Pseudomonas stutzeri, demonstrated efficient adsorption capacities for heavy metals like Mercury, Chromium, Lead, and Copper, showcasing the practical utility of melanin’s chelating properties [48,49].

Melanin redox and metal-ion chelating activity opens the possibility for its application in chemical transduction (e.g., a sensor detecting reactive oxygen species or redox-active analytes).

3.4. Biocompatibility and Biodegradability

The intrinsic biocompatibility (i.e., the ability of a material to interact with biological systems without eliciting adverse reactions) of melanin is an attribute consistently highlighted in numerous studies [50,51,55,65,66], as its biodegradability, defined as the ability of the material to be broken down by natural biological processes, such as the action of microorganisms, over time [23,65,66].

Several oxidative pathways have been reported for the degradation of melanin polymers in physiological environments, among which are enzymatic and microbial mechanisms [24,32,60]. Reactive oxygen species can indeed cleave indole–quinone linkages in eumelanin, leading to their depolymerization into smaller, water-soluble fragments [67,68]. Enzymatic degradation has been described both in vitro and in vivo [69], particularly by peroxidases, including lignin peroxidase [70], as well as laccase-like oxidases [71], which catalyze melanin oxidation in the presence of hydrogen peroxide or molecular oxygen. Microbial degradation has also been reported, especially in fungi and soil bacteria that secrete oxidative enzymes such as laccases and manganese peroxidases, resulting in partial mineralization over periods ranging from days to weeks [72].

Both these features make melanin—alongside other natural biopolymers, as cellulose, alginate, chitosan or lignin [73,74]—well-suited for bio-based wearable electronics: studies have indeed explored the potential use of biodegradable conductive and semiconductive melanin films or scaffolds for nerve tissue engineering [50,51]. Moreover, melanin-based materials remarked their efficiency in specific drug delivery [52,53,54,55] and diagnostic applications involving direct contact with biological tissues and fluids, as well as for medical implants or transient devices where the material’s eventual breakdown and clearance from the body are desirable, reducing long-term safety concerns and the need for removal surgeries, demonstrating that melanin’s biocompatibility and biodegradability represent a crucial advantage related to these materials [23,50,55,66].

4. Examples of Melanin-Based Sensors

Several classes of melanin-based sensors are currently under development and active investigation, reflecting the growing recognition of melanins as versatile functional materials for sensing applications. These sensing systems, summarized in

Table 3. Main classes of melanin-based sensors.

Sensor Typology	Sensor Architecture	Melanin Properties	Key Advantages	Main Applications
pH sensors [15,75,76,77,78]	Extended-Gate Field-Effect Transistors with melanin layer Photolithographic patterned sensor Melanin-coated graphene electrodes	Protonation/deprotonation of functional groups pH-dependent electrical modulation	High sensitivity High biocompatibility Flexibility Low cost	Biomedical diagnostics Health monitoring Wound infection detection Advanced bioelectronics
Humidity sensors [16,19,20,75,79]	Melanin films	Hygroscopic behavior Moisture-modulated charge transport	Fast response and recovery Sustainability Biocompatibility	Environmental monitoring Health monitoring
Optical sensors [21,22,40,80,81]	Melanin nanoparticles Nanoparticle-coated optical fibers Melanin films	Broadband light absorption	High sensitivity Tunable optical response Biocompatibility	Bioimaging Clinical diagnostics Industrial monitoring
Photoacoustic Sensors [17,82,83]	Melanin nanoparticles	Efficient photothermal conversion	High contrast Deep tissue penetration	Imaging Biomedical diagnostics
Metal-Melanin Sensors [18,84,85,86]	Melanin-coated metal/metal-oxide nanoparticles	Metal chelation and redox activity	Enzyme-free sensing	Biomedical diagnostics Environmental monitoring

, exploit the distinctive physicochemical properties of melanins, and explore potential applications across different sensing platforms. In the following sections, we analyze in detail the main categories of melanin-based sensors, with particular emphasis on pH, humidity, and optical sensors, highlighting their operating principles, material design strategies, and representative performance characteristics.

4.1. pH Sensors

As pH sensors are widely employed in fields such as biochemistry and medical diagnostics, their continued development is crucial [15,87,88,89]. In recent years, growing emphasis has been placed on wearable sensors, particularly those integrating pH sensors with biomarker detectors, developed for continuous and non-invasive health monitoring [87,88,89]. In this sense, melanin has emerged as a highly attractive material, as demonstrated by multiple examples, summarized in

Table 4. Melanin-based pH sensors.

Sensor	Melanin Source	Melanin Form	Other Material(s) Present	Fabrication Technique(s)
EGFET sensor [15]	synthetic	thin film	indium tin oxide gold	spin coating
sulfonated-eumelanin sensor [75]	synthetic	thin film	glass	spin coating photolithography
graphene–melanin sensor [76]	synthetic	layer	graphene	in situ polymerization
graphene–melanin sensor [77,78]	synthetic	thin film	graphene DMSO silver ink polyethylene terephthalate	screen printing

, due to its above-mentioned features, such as the ability to modulate its electrical properties in response to pH variations, since some of its functional groups can be protonated or deprotonated depending on pH value. This feature is suitable for integration into bioelectronic devices, including Extended-Gate Field-Effect Transistors (EGFETs) for pH sensing applications [15,90,91].

In the last decade, pH sensors based on EGFET architectures employing melanin thin films as the active sensing layer have indeed been developed [15]. The melanin films, synthesized from different precursors and deposited on conductive substrates such as indium tin oxide and gold, have been extensively characterized using atomic force microscopy (AFM) and electrochemical impedance spectroscopy. Device testing across a broad pH range (2–12) revealed sensitivities between 31.3 and 48.9 mV/pH, depending on both the melanin precursor and the substrate. The sensing mechanism has been attributed to specific binding sites in melanin, particularly hydroxyl groups and quinone imine moieties. In addition, sensor performance was strongly influenced by deposition conditions, substrate choice, and surface roughness, with increased roughness providing a larger effective sensing area and enhanced sensitivity. The integration of melanin into EGFETs represents a promising avenue for developing sensitive, biocompatible, and versatile pH sensors, particularly for applications requiring non-invasive or continuous monitoring in biological systems. Continued research into melanin’s properties and its integration with advanced device architectures will likely lead to further improvements in sensor performance and expand its application scope.

Beyond EGFET-based configurations, the integration of synthetic eumelanin into robust solid-state platforms using standard microfabrication techniques represents a significant advance toward scalable and miniaturized wearable devices. In particular, the successful use of conventional photolithography to pattern sulfonated-eumelanin thin films demonstrates that the material’s fundamental indole-based chemical structure, film topography, and charge-transport behavior remain unaffected by standard processing steps, as confirmed by a comprehensive characterization, by AFM, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), combined with direct and alternating current electrical measurements. Interdigitated electrode architectures fabricated with these films enabled the development of compact multisensory platforms capable of both pH and humidity sensing. Within physiologically relevant conditions (pH 5–8), sulfonated-eumelanin-based pH sensors exhibited sensitivities of approximately 50.1 mV·pH⁻¹ and demonstrated stable operation over time, highlighting their suitability for applications such as wound-healing monitoring [75].

Moreover, a particularly promising direction involves the integration of melanin with graphene-based materials, increasingly applied due to their adjustable properties and ease of functionalization, in flexible and wearable biomedical sensors to monitor physiological signals and biochemical parameters [92,93,94]. These derivatives, including graphene oxide and reduced graphene oxide, offer outstanding electrical conductivity, high carrier mobility, large surface area, and easy functionalization, making them ideal for flexible and wearable biomedical sensors [76,77,78]. Hybrid graphene–melanin systems have been shown to enhance charge transport and sensing performance through strong synergistic interactions. In particular, polymerization of eumelanin precursors in the presence of graphene-like layers (Figure 3) leads to tighter molecular packing and improved electrical conductivity, providing an effective strategy for tuning bioelectronic properties [76].

In addition, printable and flexible graphene–melanin pH sensors have been successfully fabricated using scalable techniques such as screen printing. In these devices, a conductive carbon or graphene layer is coated with a pH-responsive DMSO–melanin film (Figure 4) that serves as the active sensing element. Variations in hydrogen ion concentration modulate the electrochemical properties of the melanin layer, which are transduced by the graphene electrodes. These sensors have demonstrated high sensitivity (up to 62 ± 7 mV/pH), excellent reproducibility, and reliable operation within physiologically relevant pH ranges, making them well suited for point-of-care diagnostics, biotechnology, and continuous health monitoring [77,78].

Graphene–melanin pH sensors have also been validated in complex biological environments, including bacterial culture media, where they accurately monitored pH variations during microbial growth. Their robustness and repeatability highlight their potential for applications such as bioreactor monitoring. Moreover, the ability of these sensors to continuously and in real time track pH fluctuations makes them strong candidates for wound-care applications, where pH is a key indicator of infection and healing status [76,77,78].

Furthermore, additive manufacturing techniques such as screen printing, inkjet printing, and 3D printing could enable the precise placement of materials to form components on various flexible substrates, such as plastics, paper, cloth, and even skin, so to broaden the application landscape of melanin-based pH sensing technologies. [95,96,97].

4.2. Humidity Sensors

Melanins hold potential in humidity sensing applications (see

Table 5. Melanin-based humidity sensors.

Sensor	Melanin Source	Melanin Form	Other Material(s) Present	Fabrication Technique(s)
dopamine–melanin sensor [16,79]	synthetic	thin film	gold silicon wafer	self-assembly solution casting and spin-coating
sulfonated-eumelanin sensor [75]	synthetic	thin film	glass	spin coating photolithography
black soldier fly–melanin sensor [19]	natural	thin film	silver ink silver electrode polyethylene naphthalate	inkjet printing
graphene–melanin sensor [20]	synthetic	melanin mat	silk nanofibers graphene	immersion

for examples) due to their hygroscopic nature and their ability to respond to changes in moisture content, which lead to measurable modification in their electrical or optical characteristics [19,29,43,59]. Melanin’s humidity-sensing capabilities are due to its ability to absorb and interact with water molecules [16,19,79]. This interaction can change melanin’s charge-transport properties, leading to measurable variations in electrical resistance, impedance, or capacitance, detectable by an appropriate sensor [43,98].

High-performance humidity sensors based on self-assembled dopamine-melanin thin films have been reported by Hong and coworkers [16], in which films are formed at the air–solution interface during dopamine polymerization and subsequently transferred onto a variety of substrates. The film thickness can be precisely controlled at the nanometer scale by tuning the polymerization time, while atomic force microscopy reveals smooth and compact surface morphologies. These sensors display ultrasensitive, linear, and reproducible responses across the full relative humidity range (0–100% RH), with electrical resistance decreasing by several orders of magnitude as humidity increases. Their response and recovery times are on the order of only a few seconds (typically below 3 s and 7 s, respectively) outperforming many conventional inorganic humidity-sensing materials. The pronounced sensitivity to moisture further enables unconventional applications such as fingertip position detection, highlighting the potential of dopamine-melanin thin films for touchless interfaces, artificial skin, and bionanoelectronic sensing devices.

Further advances have been achieved by the same group, through the development of water-soluble dopamine–melanin, obtained by controlling the initial pH of the dopamine solution. This approach enables the fabrication of dense, uniform nanoscale films via solution processing, which is critical for thin-film bioelectronics. Humidity sensors based on these films exhibit exceptional performance, with resistance changes exceeding five orders of magnitude across the full humidity range and ultrafast response and recovery times below 0.5 s, significantly outperforming conventional inorganic devices [79].

Additionally, a recent study has also demonstrated as a synthetic sulfonated eumelanin derivative can be robustly integrated, through photolithography, into solid-state electronic devices. The humidity sensing measurements performed with these readily prepared bio-inspired sensors revealed conductivity changes spanning up to five orders of magnitude between dry and hydrated states, along with rapid response and recovery when exposed to human breath, underscoring the versatility of eumelanin for physiological monitoring applications [75].

In parallel, the integration of melanin into flexible and printable sensor platforms has emerged as a key strategy for advancing sustainable electronics. In particular, inkjet-printed melanin-based humidity sensors can combine high sensing performance with reduced environmental impact. Figure 5 shows a notable example is the development of flexible humidity sensors fabricated by inkjet printing biobased melanin derived from black soldier fly (BSF) waste onto interdigitated silver electrodes. The addition of choline chloride to the melanin ink increases ionic conductivity and enhances sensitivity while reducing hysteresis. These devices exhibit fast detection and recovery times (~0.8 s) and a strong impedance response, with up to a 170-fold change between 30% and 90% RH. Although prolonged exposure to high humidity can reduce sensitivity, stable long-term operation is achieved, and the sensors’ low embodied energy, substrate reusability, and potential biodegradability make them attractive for wearable, agricultural, and healthcare applications [19].

The versatility of melanin-based humidity sensing has recently been extended to multifunctional wearable systems (Figure 6). Ultrathin electronic tattoos composed of melanin-doped silk nanofibers combined with graphene exploit melanin’s humidity-responsive conductivity for skin hydration and breath sensing, while also enabling light and UV detection. These devices conform closely to the skin, maintain breathability and biocompatibility, and allow long-term wear, demonstrating the potential of melanin-based materials for imperceptible, real-time physiological and environmental monitoring [20].

Overall, both biosourced and synthetic melanin offers a sustainable, biocompatible, and versatile platform for advanced humidity-sensing applications, enabling the development of high-performance sensors [6,19,98].

4.3. Optical Sensors

Exploiting optical sensors, it is possible to detect changes in light properties when an analyte is present [22,99]. Melanin-based materials, such as melanin films, can offer improvement to this kind of device due to their unique surface chemistry, broadband photodetection, and high electrical conductivity [21,22].

The use of melanin in bio-integrated optoelectronics is indeed another significant development (see

Table 6. Melanin-based optical sensors.

Sensor	Melanin Source	Melanin Form	Other Material(s) Present	Fabrication Technique(s)
silicon–melanin sensor [21,40]	synthetic	nanoparticles	silicon silk indium tin oxide polyethylene terephthalate	3D printing dispersion
graphene–melanin sensor [22]	synthetic	nanoparticles	graphene quantum dots	in situ polymerization
melanin/porous silicon heterojunctions [81]	synthetic	film	silicon wafers indium tin oxide	deposition Metal-Assisted Chemical Etching

for significant examples). For example, the incorporation of melanin nanoparticles into silk-protein hydrogel forms a biocompatible, semiconducting layer suitable for bioimage sensors [21,40]. Such devices can interface with biological tissues, offering opportunities in clinical diagnosis and therapy; however, they are particularly sensitive to moisture due to the presence of melanin, and consequently changes in humidity influence the sensors’ performance [21,40].

A particularly significant advance (Figure 7) is represented by the incorporation of melanin into bio-integrated optoelectronic systems. For example, melanin nanoparticles embedded in silk-protein hydrogels form biocompatible, semiconducting layers suitable for bioimage sensors and wearable optoelectronics [21]. In these hybrid materials, the hydrated silk matrix enhances ionic conductivity and facilitates efficient charge transport within the melanin network. When interfaced with silicon, these composites form bio–semiconductor junctions capable of broadband photodetection, exhibiting uniform photoresponse from ultraviolet to near-infrared wavelengths. The resulting devices show fast photo-switching behavior driven by effective charge accumulation and transport at the bio-semiconductor/Si interface. Notably, their optoelectronic performance is strongly influenced by ambient humidity, as water uptake in the melanin-containing hydrogel modulates charge transport, rendering these systems inherently sensitive to moisture.

Building on this concept, fully biomaterial-based optoelectronic skins have been developed by the same group using melanin nanoparticles embedded in silk protein hydrogels. These platforms combine transparency, flexibility, and skin compatibility with photoresponsive and p-type semiconducting behavior that is further enhanced under illumination. In addition to photodetection, such optoelectronic skins exhibit self-healing properties and multifunctionality, enabling operation as strain, humidity, and UV light sensors, as well as image pixels that convert light patterns into electrical signals. These features highlight the potential of melanin–silk systems for artificial electronic skin, bio-integrated sensing, and next-generation wearable biomedical devices [40].

Beyond bio-integrated optoelectronics, melanin has also been successfully combined with nanostructured photonic materials to achieve highly selective optical chemosensing. For instance, melanin nanoparticles integrated with nitrogen-doped graphene quantum dots and embedded within molecularly imprinted polymers have been employed as ultrasensitive fluorescence probes for the selective detection of prometryn among triazine herbicides [22]. In this hierarchical nanocomposite, melanin provides bio-inspired affinity, graphene quantum dots act as efficient fluorescence quenchers, and the molecularly imprinted polymer ensures molecular selectivity. The resulting optosensor demonstrates high sensitivity, excellent selectivity, and reliable performance in real samples, offering a cost-effective and environmentally friendly alternative to conventional analytical techniques.

Moreover, melanin-inspired oligomeric derivatives have been theoretically investigated as optical sensing materials for the selective detection of polynitroaromatic compounds. Computational studies indicate strong and reversible interactions between these bioinspired materials and dinitro- and trinitroaromatic analytes, leading to significant modulation of electronic, optical, and vibrational properties. While analyte-dependent absorption shifts are not always consistent, bandgap reductions suggest promising routes for luminescence-based detection, positioning melanin-inspired compounds as sustainable platforms for optical and optoelectronic sensing of toxic nitroaromatic compounds [80].

Melanin has also been integrated with inorganic semiconductors to form hybrid optoelectronic sensors. In particular, melanin/porous silicon heterojunctions have been developed as multifunctional platforms for photodetection and chemical sensing. These diode-like structures exhibit rectifying behavior and have been demonstrated to detect analytes such as glucose and volatile organic compound vapors without the need for additional biological recognition elements. Although their photovoltaic performance remains limited, such heterojunctions are well suited for incorporation as embedded photodetectors within sensor chips, combining optical and chemical sensing capabilities in a biocompatible architecture [81].

Overall, these studies demonstrate that melanin and melanin-inspired materials constitute a versatile and powerful class of optical sensing materials. Their integration into bio-integrated optoelectronics, nanophotonic composites, and hybrid semiconductor devices enables sensitive, selective, and multifunctional optical sensors, with strong potential for applications in environmental monitoring, food safety, healthcare diagnostics, and wearable and implantable bioelectronics.

4.4. Other Melanin-Based Sensors

Melanin nanoparticles are also useful in further sensor-related applications, as summarized in

Table 7. Other melanin-based sensors.

Sensor	Melanin Source	Melanin Form	Other Material(s) Present	Fabrication Technique(s)
whole-cell sensor [17]	natural	nanoparticles	/	cell culture
E. coli–melanin nanoparticles [82]	natural	nanoparticles	/	biosynthesis
melanin–silver nanoparticles [84]	synthetic	nanoparticles	silver silica	in situ polymerization
melanin–Prussian blue derivatives [18]	synthetic	nanoparticles	Fe3+, Cu2+, or Ni2+ Prussian blue derivatives	capping
melanin-coated quartz crystal microbalance sensors [9]	natural	film	quartz gold	coating
melanin-coated electrodes [85]	synthetic	film	glassy carbon indium tin oxide gold	electrochemical deposition
a molecularly imprinted melanin [86]	synthetic	film	glassy carbon	electrochemical polymerization

. Among them, there is photoacoustic imaging, which combines the deep penetration of ultrasonic imaging with the high specificity of optical imaging, while being non-invasive [27], since they can convert light into acoustic waves, which are then detected by a photoacoustic sensor [82]. Recently, the development of optical ultrasound sensors has brought some advantages, including high sensitivity, broad bandwidth, and miniaturization [27]. Owing to their high photothermal conversion efficiency, melanins represent an attractive alternative to noble metal nanoparticles, whose photoacoustic efficiency can be limited by scattering losses and thermal dissipation [27,82,100].

Biomimetic strategies have leveraged melanin’s intrinsic optical functionality at the cellular level. A notable example is the development of whole-cell photoacoustic sensors based on pigment relocalization in melanophore cells. In this system, melanin-filled melanosomes dynamically redistribute within cells in response to ligand binding to G protein–coupled receptors, producing detectable changes in photoacoustic signal amplitude and frequency. This approach represents the first photoacoustic sensor for GPCR-mediated signaling and establishes a generalizable framework for multiplexed molecular sensing using mobile endogenous chromophores [17].

Advances in biosynthetic strategies have enabled scalable production of melanin nanoparticles with tailored optoelectronic properties. Multi-enzyme biosynthesis in Escherichia coli has yielded melanin nanoparticles with enhanced polymerization, stronger near-infrared absorption, and high photothermal conversion efficiency (~66.5%), resulting in improved photoacoustic imaging performance and effective photothermal therapy in breast tumor models. Such biosynthetically regulated systems provide a safe and tunable platform for future cancer diagnostics and therapy [82].

Hybrid melanin–metal nanostructures have also been engineered to further enhance photoacoustic contrast. Ternary core–shell nanoparticles composed of silver cores, DHICA-derived melanin, and a silica matrix demonstrate that synergistic interactions at the metal–polymer interface significantly amplify photoacoustic signals. Precise control over silver content and particle size enables optimization of optical absorption and thermal confinement, while the silica shell stabilizes the structure and prevents aggregation. These biocompatible hybrid nanoparticles show strong potential not only for high-performance photoacoustic imaging but also for combined photothermal therapy and theranostic applications [83].

Moreover, the high affinity of melanin-based materials for metal ions enables the sustainable surface modification of metal and metal-oxide nanoparticles, producing multifunctional biohybrid platforms with magnetic, photothermal, catalytic, antimicrobial, and antioxidant properties [84]. These characteristics are particularly valuable for biosensing, bioimaging, and theranostics, although challenges remain in achieving scalable synthesis, precise control of physicochemical properties, and predictable in vivo behavior.

Melanin–metal hybrids have been successfully applied to nonenzymatic electrochemical sensing. For example, melanin-supported Prussian blue nanoparticles doped with Fe³⁺, Cu²⁺, or Ni²⁺ have been developed as highly sensitive amperometric sensors for hydrogen peroxide. In these systems, melanin serves as an eco-friendly and stable support while enhancing electron transfer and long-term stability. The resulting sensors exhibit low detection limits, wide linear ranges, and successful application in real samples, such as tooth-whitening gels, effectively mimicking enzymatic activity without the associated instability of biological catalysts [18].

Melanin-based materials have also proven highly effective for heavy metal detection using diverse transduction mechanisms. Tea-melanin-coated quartz crystal microbalance sensors represent the first example of piezoelectric platforms capable of real-time, highly sensitive metal ion detection [9]. These systems show exceptional sensitivity to Hg²⁺ ions, with detection limits down to the ppb range, and selective responses to a wide variety of metal ions. Structural and conformational changes in melanin upon metal binding, confirmed by AFM, circular dichroism, and infrared spectroscopy, underpin the sensing mechanism and highlight the suitability of quartz crystal microbalance as a tool for probing melanin–metal interactions. [9,85].

Complementary electrochemical approaches have employed synthetic melanin nanoparticle–coated electrodes for voltammetric detection of heavy metals such as Pb, Cd, Co, Zn, Ni, and Fe. Melanin films deposited in situ on conductive substrates exhibit stable, reproducible, and highly sensitive responses, with particularly strong selectivity toward ferric ions. These platforms demonstrate excellent linearity and low detection limits, supporting their use in environmental monitoring of water contaminants. [85].

Beyond metals, melanin-based sensors have shown promising performance in the detection of several pharmaceutical products, pesticides, and related contaminants, as thoroughly reported in a recent review by Pane et al. [8]. In this regard, a new electrochemical sensing strategy based on the tunable electropolymerization of dopamine in the presence of bentazon has been developed [86]. Bentazon, an organophosphorus pesticide widely used for insect control, was employed as a template molecule to fabricate a molecularly imprinted polymer on a glassy carbon electrode (Figure 8). During electropolymerization, melanin forms a tailor-made film containing cavities that match the size and stereochemical features of bentazon, resulting in a highly selective recognition interface. Detection is achieved using ferricyanide as a redox probe, with signal changes governed by occupancy of the imprinted cavities. The resulting sensor exhibits remarkable sensitivity, enabling bentazon detection in the femtomolar range, along with excellent selectivity against potential interferents. Importantly, the platform was successfully applied to real water samples without any sample pretreatment, clearly discriminating bentazon-contaminated samples from controls. This approach highlights the broad applicability of dopamine electropolymerization for constructing highly selective and sensitive sensors, which can be readily adapted to detect other pesticides or small-molecule targets by simply changing the template molecule.

5. Importance and Potential Impacts

Lately, melanin-based sensors are gaining significant attention in scientific research due to the unique physicochemical properties of these biopolymers, among which is an inherent adhesiveness [8]. This, combined with their biocompatibility and versatility, makes them ideal for surface modification and the fabrication of electrochemical sensors. Furthermore, the ease of preparation and modification enhances their appeal in sustainable technological development [19,23,28,29]. Melanin-based sensors can detect environmentally relevant targets, and nanoparticles can be incorporated into environmental sensors to detect toxins, heavy metals, and organic pollutants [22,81]. Due to their ability to undergo reversible redox reactions, melanins can interact with various analytes, transducing chemical signals into measurable electrical responses [47].

In biomedical applications, melanin-based wearable biosensors are revolutionizing healthcare by enabling continuous health monitoring, early disease detection, and personalized medicine [21,40,76,81].

Melanin-based sensors are also exploited in chemical and biological sensing, detecting substances in the body or the environment. For example, glucose sensing is useful for people affected by diabetes, whereas cortisol monitoring provides insight into a person’s stress level [81,101]. Such sensors can facilitate early diagnosis and notify emergency services when critical health data are detected [102].

To create ultrathin interfaces with biological matter, two-dimensional materials can be used, and melanin could be integrated with 2D material-based electronics to enhance the sensor’s performance [76,77,78,86]. In addition, melanin offers environmental sustainability benefits as a biomaterial derived from natural sources [19,24]. Moreover, melanin’s preparation is easy and environmentally friendly, which makes it even more appealing in the biosensors field [19,34,35].

Furthermore, melanin is optimal for optical sensing, due to its photoabsorptive behavior [21,38,40,41]. The interplay between plasmonic fields and optical properties represents another area where melanin’s optical characteristics might be exploited [83,103]. In conclusion, melanin’s properties and its integration into various sensor architectures promise significant advancements in creating more effective, sustainable, and user-friendly sensing solutions for the future.

6. Challenges and Limitations

Despite the several advantages they offer, melanin-based sensors face significant challenges that impede their widespread application, mainly associated with their chemical and structural complexity. A major challenge, indeed, involves reproducibility and standardization, stemming from the inherent structural heterogeneity of both natural and synthetic melanin, in terms of monomer composition, oxidation levels, and impurity content [19,28,31]. This intrinsic variability compromises the reliability and comparability of sensing outcomes, thereby hindering the consistent sensing performance across different production batches [8,28]. Sensitivity, selectivity, and the potential for cross-interference represent further hurdles: multiple mechanisms—not yet fully understood—including surface binding, changes in oxidation state, hydration, and ionic or electronic conduction, all contribute to melanin’s sensing properties [98,104]. Moreover, melanins’ various binding sites for protons, metal ions, and small molecules increase the risk of cross-sensitivity [8,46].

The relative influence of these mechanisms can vary significantly with environmental conditions: parameters such as pH, humidity, and ionic strength can therefore affect the signal for a target analyte [19,21,40]. Processes such as moisture-induced swelling, or slow charge diffusion, potentially coupled with redox chemistry, can contribute to slow sensor kinetics, hysteresis, and signal drift, thereby exerting a critical influence on both sensors’ response and recovery time [21,40,98]. Consequently, achieving long-term stability under harsh environmental conditions remains a significant challenge for these sensors [8,37].

An additional constraint lies in melanin’s relatively low electronic conductivity: although melanin exhibits mixed ionic and electronic conduction, its electrical conductivity remains substantially lower compared to that of conventional inorganic semiconductors or metals [98]. This constraint affects its performance in applications relying on efficient electron transfer—such as advanced biosensors for pathogen detection—where fast response times and low-noise signal processing are essential. While enhancement strategies through composite formation, doping, or morphological engineering can be exploited to improve conductivity, they often introduce additional complexity to the fabrication process [8]. Scalability, cost, and processing represent additional challenges, since obtaining high-purity, consistent-quality melanin, processing it into uniform thin films or nanoscale structures, and integrating them with electrodes demand significant engineering efforts.

Mechanical robustness and integration are also crucial: in the case of wearable, flexible, or implantable devices, the sensing layer must withstand mechanical stress and stable electrical contacts [19,95,96,97]. Fabrication techniques must indeed balance cost-effectiveness, throughput, and performance for widespread adoption. Overcoming these limitations will be vital for melanin-based sensors to realize their full potential across a range of advanced sensing applications.

7. Future Directions

Melanin-based sensors are an emerging area of research, exploiting melanin’s properties for various purposes [19,92,95]. The future directions in this field include enhancing the electrical conductivity and optoelectronics properties of melanin-based materials, improving the fabrication of flexible and wearable electronics, expanding the range of detectable analytes, and improving the integration into sophisticated biosensing platforms for different applications ranging from medical diagnostics to environmental monitoring [7,50,55,66,80,81].

Melanin can be synthesized via oxidative polymerization of dopamine or extracted from plants, animals, bacteria, and fungi [24,25]. Both natural and synthetic melanin exhibit intrinsic biocompatibility, antioxidant activity, free-radical scavenging, and metal ion chelation [48,60,61,62,63,64]. Furthermore, melanin can be incorporated into conductive polymers, forming a material with tunable properties, which makes it useful for sensing in clinical, food quality, and environmental monitoring applications [44].

A fundamental future direction is the development of advanced fabrication techniques. These materials can be obtained through simple, green procedures, making them sustainable and economic [24,34,35]. A screen-printing process has indeed been employed to fabricate an electrochemical pH sensor based on graphene and melanin [77]. Another promising way of sensor preparation could contemplate the integration of melanin-like nanomaterials with laser-induced graphene, aiming to obtain a sensor with tunable surface chemistry, high electrical conductivity, mechanical flexibility, and biocompatibility [105].

Moreover, expanding the range of analytes detectable by melanin-based sensors is also a key focus. Some researchers are trying to obtain sensors for nitroaromatic compounds, which are toxic and dangerous for the environment [80]. Melanin has also been studied to engineer metal and metal-oxide nanoparticles for groundbreaking biomedical applications, including diagnostics and therapeutics [84]. Integration of melanin-based materials into advanced biosensing platforms is also gaining traction. For example, biomimetic melanin-like deposition has been used to enhance the sensitivity of sandwich-type electrochemiluminescence (ECL) immunosensors [106]. Since melanins present a lot of useful properties, optimal in the biosensor field, in the future they can possibly be integrated into microelectromechanical systems (MEMS) [107]. Other properties are still being studied, such as the ability of melanin-like nanoparticles to adsorb light and convert it into heat [108]. Another exciting development is represented by the use of melanin bio-inks on paper-based laser-induced graphemic electrodes for the detection of neurotransmitters [109]. In conclusion, the focus remains on leveraging melanin’s natural advantages while addressing existing challenges to realize its full potential in next-generation sensing technologies.

8. Conclusions

This review has critically examined melanin and melanin-derived materials as emerging platforms for sensing technologies, highlighting both their multifunctional physicochemical nature and the underlying mechanisms that govern their sensing behavior. A key insight is that melanin’s sensing performance is not attributable to a single dominant property, but rather to the complex interplay between hydration-dependent charge transport, redox activity, molecular binding affinity, and supramolecular organization. While this multifunctionality enables applicability across chemical, biological, optical, and environmental sensors, it also introduces intrinsic complexity that must be carefully managed in device design.

Despite a growing number of compelling proof-of-concept demonstrations, the translation of melanin-based sensors from laboratory studies to robust, real-world technologies remains challenging. Structural heterogeneity, batch-to-batch variability, and incomplete control over oxidation state and morphology continue to limit reproducibility and comparability across studies. Moreover, the strong sensitivity of melanin to environmental factors such as humidity, pH, and ionic strength—while advantageous for certain sensing modalities—can lead to signal drift, cross-sensitivity, and long-term instability if not deliberately engineered. These issues underscore the need for standardized synthesis protocols, rigorous materials characterization, and clearer correlations between molecular structure, processing conditions, and device-level performance.

Another critical limitation concerns electrical performance. Although melanin functions as a mixed ionic–electronic conductor, its relatively low electronic conductivity constrains applications that require fast response times, low noise, or high-frequency operation. Strategies based on hybridization with conductive fillers, nanostructuring, or molecular engineering have shown promise, but they often increase fabrication complexity and may compromise sustainability or biocompatibility if not carefully optimized.

Looking forward, progress in this field will depend on a transition from largely empirical demonstrations toward mechanism-driven design principles. Future research should prioritize: (i) the development of chemically defined and reproducible melanin analogues; (ii) advanced in situ and operando techniques to disentangle ionic, electronic, and redox contributions during sensing; (iii) scalable and low-energy fabrication routes compatible with flexible and wearable electronics; and (iv) system-level integration with data processing, multiplexed architectures, and closed-loop sensing platforms. Computational modeling and machine-learning-assisted materials discovery may also play a pivotal role in accelerating rational design and performance optimization.

In conclusion, melanin-based materials occupy a unique position at the intersection of bioinspired chemistry, sustainable materials science, and next-generation sensing. Their inherent ability to couple chemical, optical, and electrical responses within a single biopolymeric platform is particularly compelling for biosensing applications, where biocompatibility, multifunctionality, and environmental adaptability are essential. While substantial challenges remain, addressing them through coordinated interdisciplinary efforts could transform melanin from a versatile laboratory material into a reliable and scalable component of future biomedical, environmental, and industrial sensing systems.