Formation of Homogeneous Epinephrine-Melanin Solutions to Fabricate Electrodes for Enhanced Photoelectrochemical Biosensing

The "Unconventional" Effect of Cysteine on the In Vitro Synthesis of Melanin

This report details some of our observations regarding the impact of cysteine on the air-mediated oxidation of catecholamines, particularly epinephrine. The intent was to synthesize light-colored, pheomelanin-like materials. Pheomelanin is commonly described as a material generated from a mixture of catecholamines and cysteine. However, we observed that (1) the presence of cysteine resulted in a concentration-dependent delay in the onset of color formation and (2) the presence of cysteine resulted in darker, more eumelanin-like materials. These effects were particularly impactful in the case of epinephrine. More elaborate studies involving other amino acids or scaled-up reactions were conducted with epinephrine as the precursor. These studies show that other amino acids, e.g., methionine or serine, could lead to darker materials, but none were as impactful as cysteine. Although our results are in contrast to typical descriptions regarding the impact of cysteine on the synthesis of melanin, they may reflect crucial differences between the in vitrovsin vivo synthesis of pheomelanin.

Experimental Methods to Get Polydopamine Films: A Comparative Review on the Synthesis Methods, the Films' Composition and Properties

In 2007, polydopamine (PDA) films were shown to be formed spontaneously on the surface of all known classes of materials by simply dipping those substrates in an aerated dopamine solution at pH = 8.5 in the presence of Tris(hydroxymethyl) amino methane buffer. This universal deposition method has raised a burst of interest in surface science, owing not only to the universality of this water based one pot deposition method but also to the ease of secondary modifications. Since then, PDA films and particles are shown to have applications in energy conversion, water remediation systems, and last but not least in bioscience. The deposition of PDA films from aerated dopamine solutions is however a slow and inefficient process at ambient temperature with most of the formed material being lost as a precipitate. This incited to explore the possibility to get PDA and related films based on other catecholamines, using other oxidants than dissolved oxygen and other deposition methods. Those alternatives to get PDA and related films are reviewed and compared in this paper. It will appear that many more investigations are required to get better insights in the relationships between the preparation method of PDA and the properties of the obtained coatings.

Synthesis of soluble melanin nanoparticles under acidic conditions using Burkholderia cepacia tyrosinase and their characterization

Melanin nanoparticles (MNPs) used for biomedical applications are often synthesized via the chemical auto-oxidation of catecholic monomers such as dopamine and 3,4-dihydroxyphenylalanine (DOPA) under alkaline conditions. However, the synthetic method for the chemical synthesis of MNP (cMNP) is relatively straightforward and more robust to control their homogenous particle size and morphology than the corresponding enzymatic synthetic methods. In this study, we demonstrated that the simple enzymatic synthesis of MNPs (eMNPs) with homogenous and soluble (<20 nm diameter) properties is possible using dopamine and Burkholderia cepacia tyrosinase (BcTy) under acidic conditions (i.e., pH 3.0). BcTy was highly reactive under pH 5.0, where the natural and chemical oxidation of catechol is complex, and thus melanin was synthesized via the hydroxylation of phenolic substrates. The detailed chemical analysis and characterization of the physical properties of the eMNPs confirmed the higher preservation of the catechol and primary amine moieties in the monomer substrate such as dopamine under acidic conditions. The eMNPs showed enhanced antioxidant activity and conferred stickiness to the formed hydrogel compared to the chemical auto-oxidation method owing to the large number of hydroxyl groups remaining such as catechol and quinone moieties. Because of these advantages and characteristics, the synthesis of MNPs using BcTy under acidic conditions can open a new path for their biomedical applications.

Melanin nanoparticles (MNPs) used for biomedical applications are often synthesized via the chemical auto-oxidation of catecholic monomers such as dopamine and 3,4-dihydroxyphenylalanine (DOPA) under alkaline conditions.

Functional polymers in photoelectrochemical biosensing

Photoelectrochemical (PEC) analysis is a detection technique that has gained a wide attention in sensing applications. PEC presents the advantages of high sensitivity, low background signal, simple equipment and easy miniaturization. In PEC detection, light is used as an excitation source while current or voltage is measured as the output detection signal. The ability to couple the PEC process with specific bioreceptors gives PEC biosensing a unique advantage of being both selective and sensitive. The growing interest in PEC bioanalysis has resulted in essential progress in its analytical performance and biodetection applications. Functional polymers have different applications in the development of novel PEC biosensing platforms. Recently, the interest in polymer-based photoactive materials has emerged as they are efficient and less toxic alternatives to certain kinds of inorganic semiconductors and sensitizers. Moreover, molecularly imprinted polymers are a class of synthetic bioreceptors that are increasingly used in PEC bioanalytics. In this review, we will provide an overview on functional polymer-based PEC biosensing approaches. Novel classes of polymers as photoactive materials are reviewed and selected applications are described. Furthermore, molecularly imprinted polymers in the development of smart and sensitive PEC bioanalytical strategies are discussed.

Spectroscopic Investigation and Nanoscale Characterization of Epinephrine Autooxidation under Alkaline Conditions

Melanins are intriguing biomaterials with unique physical and chemical properties. Due to the insoluble nature of the synthetic melanins prepared from different precursors, such as 3,4-dihydroxy-phenylalanine (DOPA) and dopamine (DA), it is still challenging to reveal the structure-property relationships. In this work, the autoxidation of epinephrine (EP) under basic conditions was investigated from the perspective of supramolecular chemistry, and the formed soluble epinephrine-melanin (EPM) was characterized on the nanoscale. The supramolecular aggregate nature of oxidation products has been identified on the basis of spectroscopic investigations. A two-dimensional sheet-like morphology with highly ordered in-plane stacking structures was observed for the first time, and the thickness of the nanosheet increased with increasing EPM concentration. More importantly, in contrast to the well-known monotonic absorption profiles of synthetic melanins, EPM shows featured and unusual pH-responsible absorption profiles in the near-ultraviolet region (UVA). The decrease in pH can induce the disappearance of the absorption in the lower-energy band and the reduction of aggregate size. The oxidative and aggregation kinetic processes of EP were investigated in three different alkaline systems by the combination of absorption and fluorescence spectroscopies. The oxidation process of EP shows concentration- and buffer-dependent behaviors. The unusual absorption properties of EPM were exploited for the fabrication of transparent UV-shielding chitosan biofilms and gelatin hydrogels. Extensive research on the molecular structures, supramolecular exciton coupling, and material-oriented property exploitation of EPM is highly anticipated.

Polydopamine-PEG-Folic Acid Conjugate Film Engineered TiO2 Nanotube Arrays for Photoelectrochemical Sensing of Folate Binding Protein

Serum-soluble folate binding protein (FBP) is an important tumor marker, and the development of a simple biosensing method is highly needed. In this work, a photoelectrochemical (PEC) biosensor for the detection of FBP was proposed based on the construction of an antifouling interface and the unique ligand-protein recognition. The PEC sensing platform was prepared by the biomimetic polydopamine (PDA) coating on TiO₂ nanotubes arrays (NTAs). A significant PEC enhancement effect was obtained due to the macroporous structures. Excellent antifouling performance was achieved by conjugation of amino-group-terminated 8-arm poly(ethylene glycol) (PEG). The incorporation of folic acid (FA) retains the antifouling property and shows recognition abilities toward FBP. The fabricated PEC biosensor shows good analytical performance. The combination of ligand-protein recognition and a PEC antifouling interface provides a good consideration for the development of FBP biosensors.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

Photoelectrochemical Biosensor for Sensitive Detection of Soluble CD44 Based on the Facile Construction of a Poly(ethylene glycol)/Hyaluronic Acid Hybrid Antifouling Interface

The serum level of soluble CD44 is directly associated with several clinicopathological parameters of malignant diseases. There is a great need for the development of an easy and cost-effective detection method for soluble CD44 for both the diagnosis and the treatment of cancers. In this work, a simple photoelectrochemical (PEC) method for the sensitive detection of serum-soluble CD44 is proposed based on the construction of a hybrid antifouling coating on the TiO₂ substrate. Hyaluronic acid (HA) and poly(ethylene glycol) (PEG) are co-immobilized using a biomimetic one-step surface functionalization approach. The immobilized HA shows strong recognition abilities toward soluble CD44, and the synergistic antifouling effect achieved by the combination of PEG and HA improves the sensing specificity. Based on the inhibitory effect of CD44 recognition on the PEC signal of the TiO₂ substrate, a PEC biosensor is developed with a wide response range and a low detection limit. The development of antibody-free biosensors may promote the application of soluble CD44 as a biomarker for the diagnosis and treatment of malignant diseases.

Photoelectrochemical detection of ultra-trace fluorine ion using TiO2 nanorod arrays as a probe

A photoelectrochemical (PEC) method based on the etching reaction of F ions on the surface of TiO₂ nanorod arrays (TNRs) was proposed for the high sensitivity and selectivity detection of F ions. With the increase of F ion concentration, the surface etching reaction on TNR becomes more intense, resulting in the increased number of surface active sites, the reduction of electron transfer resistance, and the increase of photocurrent density. The prepared TNRs as a PEC probe exhibits a good linear relationship between photocurrent increment and the logarithm of F ion concentration in the range from 0.05 to 1000 nM with an ultra-trace detection limit of 0.03 nM for F ion detection.

A photoelectrochemical (PEC) method based on the etching reaction on TiO₂ nanorod arrays is proposed for detection of F ions.

A versatile cathodic "signal-on" photoelectrochemical platform based on a dual-signal amplification strategy

Novel cathodic photoelectrochemical (PEC) aptasensors for sensitive and selective determination of thrombin and Pb²⁺ were developed based on a new dual-signal amplification strategy. The presence of gold nanoparticles (AuNPs) could quench the PEC signal of bismuth oxyiodide (BiOI). At the same time, the redox moiety G-quadruplex/hemin or ferrocene (Fc) was found to enhance the PEC signal of BiOI. So, in the presence of thrombin or Pb²⁺, the interaction between target and the aptamer resulted in the releasement of the AuNPs, as well as shorter distance between the redox moiety and the electrode surface. Hence dual-enhanced cathodic PEC biosensor strategy was realized. Under the optimized conditions, the detection limits of thrombin and Pb²⁺ were 17.3 fM and 3.16 pM, respectively with good selectivity. At the same time, the PEC performance of redox moiety G-quadruplex/hemin and Fc was compared.