A photoelectrochemical aptasensor for thrombin based on the use of carbon quantum dot-sensitized TiO2 and visible-light photoelectrochemical activity

Carbon-based photoelectrochemical sensors: recent developments and future prospects

Owing to the considerable potential of photoelectrochemical (PEC) sensors, they have gained significant attention in the analysis of biological, environmental, and food markers. However, the limited charge mass transfer efficiency and rapid recombination of electron hole pairs have become obstacles in the development of PEC sensors. In this case, considering the unique advantages of carbon-based materials, they can be used as photosensitizers, supporting materials and conductive substrates and coupled with semiconductors to prepare composite materials, solving the above problems. In addition, there are many types of carbon materials, which can have semiconductor properties and form heterojunctions after coupling with semiconductors, effectively promoting the separation of electron hole pairs. Herein, we aimed to provide a comprehensive analysis of reports on carbon-based PEC sensors by introducing their research and application status and discussing future development trends in this field. In particular, the types and performance improvement strategies of carbon-based electrodes and the working principles of carbon-based PEC sensors are explained. Furthermore, the applications of carbon-based photoelectric sensors in environmental monitoring, biomedicine, and food detection are highlighted. Finally, the current limitations in the research on carbon-based PEC sensors are emphasized and the need to enhance the sensitivity and selectivity through material modification, structural design, improved device performance, and other strategies are emphasized.

An electrochemical biosensor for the amplification of thrombin activity by perylene-mediated photoinitiated polymerization

BACKGROUND

Thrombin, a coagulation system protease, is a key enzyme involved in the coagulation cascade and has been developed as a marker for coagulation disorders. However, the methods developed in recent years have the disadvantages of complex operation, long reaction time, low specificity and sensitivity. Meanwhile, thrombin is at a lower level in the pre-disease period. Therefore, to accurately diagnose the disease, it is necessary to develop a fast, simple, highly sensitive and specific method using signal amplification technology.

RESULTS

We designed an electrochemical biosensor based on photocatalytic atom transfer radical polymerization (photo-ATRP) signal amplification for the detection of thrombin. Sulfhydryl substrate peptides (without carboxyl groups) are self-assembled to the gold electrode surface via Au-S bond and serve as thrombin recognition probes. The substrate peptide is cleaved in the presence of thrombin to generate -COOH, which can form a carboxylate-Zr(IV)-carboxylate complex via Zr(IV) and initiator (α-bromophenylacetic acid, BPAA). Subsequently, an electrochemical biosensor was prepared by introducing polymer chains with electrochemical signaling molecules (ferrocene, Fc) onto the electrode surface by photocatalytic (perylene, Py) mediated ATRP using ferrocenylmethyl methacrylate (FMMA) as a monomer. The concentration of thrombin was evaluated by the voltammetric signal generated by square wave voltammetry (SWV), and the result showed that the biosensor was linear between 1.0 ng/mL ∼ 10 fg/mL, with a lower detection limit of 4.0 fg/mL (∼0.1 fM). Moreover, it was shown to be highly selective for thrombin activity in complex serum samples and for thrombin inhibition screening.

SIGNIFICANCE

The biosensor is an environmentally friendly and economically efficient strategy while maintaining the advantages of high sensitivity, anti-interference, good stability and simplicity of operation, which has great potential for application in the analysis of complex samples.

Atomic Layer Deposition of Defective Amorphous TiOx Thin Films with Improved Photoelectrochemical Performance

A proper control of defects in TiO2 thin films is challenging work for enhancing the photoelectrochemical (PEC) efficiency in water splitting processes. Additionally, a deep understanding of how defects affect the PEC performance of TiO2 thin films is of great interest for achieving better performance. With these aims, we prepared defective amorphous TiOx thin films at various growth temperatures by atomic layer deposition using tetrakis(dimethylamido)titanium as the Ti precursor. Careful X-ray photoelectron spectroscopy and electron spin resonance spectroscopy analyses revealed that the defect concentration in the TiOx thin films can be controlled by adjusting the growth temperature during the ALD process. We also evaluated the light absorption properties of the deposited TiOx thin films using ultraviolet-visible absorption spectroscopy. And it was found that the TiOx thin film deposited at a growth temperature of 200 °C exhibited the highest defect concentration and the highest photocurrent density of 0.051 mA/cm2 at 1.23 V vs reversible hydrogen electrode (RHE) compared to those of the other films. The light absorption efficiency, photogenerated charge separation efficiency, and charge transfer efficiency of defective amorphous TiOx thin films were carefully studied to understand the correlation between the defect concentration in the prepared TiOx thin film and its PEC activity. This study provides insight into the PEC properties of defective amorphous ALD-TiOx thin films.

Molecular imprinted based microcryogels for thrombin purification

In addition to understanding and explaining the functions of proteins, the need for low-cost, easy and efficient purification methods has been increasing in the field of protein purification, which is also important for enzyme production. In this context, an alternative approach has been developed for the purification of thrombin, which has a crucial role in the hemostatic process, via thrombin imprinted microcryogels that allow reuse and have high selectivity. The characterization studies of the microcryogels were accomplished with micro-computed tomography (µCT), scanning electron microscopy (SEM), optical microscope, surface area measurements (BET analyses) and swelling test measurements. By scanning various parameters affecting thrombin adsorption, the maximum thrombin adsorption capacity (Qmax) was found to be 55.86 mg/g. Also, the selectivity of microcryogels was investigated with the competitive agents and reusability studies were performed. The purity of thrombin was evaluated by Fast Performance Liquid Chromatography (FPLC) method. Experimental results indicated that adsorption of thrombin by the developed microcryogels fit the Langmuir isotherm model (Qmax: 55.86 mg/g, R2: 0.9505) and pseudo-second order for three different thrombin concentrations (R2: 0.9978, R2: 0.9998, R2: 0.9999).

Self-powered photoelectrochemical aptasensor based on hollow tubular g-C3N4/Bi/BiVO4 for tobramycin detection

A highly sensitive photoelectrochemical aptasensor based on phosphorus-doped hollow tubular g-C₃N₄/Bi/BiVO₄ (PT-C₃N₄/Bi/BiVO₄) for tobramycin (TOB) detecting was developed. This aptasensor is a self-powered sensing system which could generate the electrical output under visible light irradiation with no external voltage supply. Based on the surface plasmon resonance (SPR) effect and unique hollow tubular structure of PT-C₃N₄/Bi/BiVO₄, the PEC aptasensor exhibited an enhanced photocurrent and favorably specific response to TOB. Under the optimized conditions, the sensitive aptasensor presented a wider linearity to TOB in the range of 0.01-50 ng mL^-1 with a low detection limit of 4.27 pg mL^-1. This sensor also displayed a satisfying photoelectrochemical performance with optimistic selectivity and stability. In addition, the proposed aptasensor was successfully applied to the detection of TOB in river water and milk samples.

Highly Sensitive Photoelectrochemical Biosensor for MicroRNA-21 Based on a Dumbbell-Shaped Heterostructure AuNRs@end-TiO2 Combined with Carbon Dots as Photosensitizers and Duplex-Specific Nuclease-Assisted Signal Amplification

TiO₂ was grown on both ends of gold nanorods (AuNRs) to form a dumbbell-shaped heterostructure (called AuNRs@end-TiO₂) first, and then assembled on the fluorine tin oxide (FTO) electrode surface through hydrophobic interactions to construct a concise photoelectrochemical microRNA-21 (miRNA-21, model target) biosensor using carbon dots (CDs) as photosensitizers. Hairpin probes (HPs) were fixed on the AuNRs@end-TiO₂-modified FTO electrode surface through the Au-S bond, and CDs-modified complementary DNA (CDs-cDNA) served as photosensitive probes. In the presence of the target, miRNA hybridized with the HP and triggered double-strand-specific nuclease to cleave the complementary part of the HP with miRNA, and miRNA was released, which can trigger another cycle to realize signal amplification. Many HPs were cleaved and the complementary sequence with cDNA was exposed, which can capture the photosensitive probes to the electrode surface and resulted in photocurrent enhancement. The photocurrent intensity system has a linear relationship with the logarithm of the miRNA concentration in the range of 0.1 fM to 100 pM with a low detection limit of 96 aM (S/N = 3). The biosensor has high sensitivity, good selectivity, and good reproducibility and shows satisfactory results in actual sample detection.

Recent Progresses in Development of Biosensors for Thrombin Detection

Thrombin is a serine protease with an essential role in homeostasis and blood coagulation. During vascular injuries, thrombin is generated from prothrombin, a plasma protein, to polymerize fibrinogen molecules into fibrin filaments. Moreover, thrombin is a potent stimulant for platelet activation, which causes blood clots to prevent bleeding. The rapid and sensitive detection of thrombin is important in biological analysis and clinical diagnosis. Hence, various biosensors for thrombin measurement have been developed. Biosensors are devices that produce a quantifiable signal from biological interactions in proportion to the concentration of a target analyte. An aptasensor is a biosensor in which a DNA or RNA aptamer has been used as a biological recognition element and can identify target molecules with a high degree of sensitivity and affinity. Designed biosensors could provide effective methods for the highly selective and specific detection of thrombin. This review has attempted to provide an update of the various biosensors proposed in the literature, which have been designed for thrombin detection. According to their various transducers, the constructions and compositions, the performance, benefits, and restrictions of each are summarized and compared.

Ultrasensitive Photoelectrochemical Biosensor for microRNA-155 Based on Energy Transfer between Au Nanocages and Red Emission Carbon Dot-Assembled Nanosheets Coupled with the Duplex-Specific Nuclease Enzyme-Assisted Target Recycling Strategy

Energy transfer (ET) is an effective tool to construct photoelectrochemical (PEC) biosensors for its high sensitivity. Since the materials to develop ET systems are limited, exploring new and universal ET systems is significant. Herein, new photoactive nanosheets (R-CDs NS) formed by self-assembling of red emission carbon dots (R-CDs) have been synthesized, which exhibit wide visible light absorption and stable photocurrent response and have an obvious sensitization effect for TiO₂. Gold nanocages (AuNCs), whose absorption overlap well with the R-CDs' emission, were synthesized and served as PEC quenchers for the photosensitized system that consists of TiO₂ and R-CDs. The ET between R-CDs and AuNCs can boost the recombination of photogenerated electron-hole pairs of R-CDs and results in a quenched photocurrent of this system. MicroRNA-155 was chosen as a model target. First, the nanocomposite containing R-CDs NS and AuNCs was prepared through DNA modification and hybridization. In the absence of the target, AuNCs and R-CDs were close enough for ET, with TiO₂-modified FTO serving as the working electrode, and a quenched photocurrent was detected. In the presence of the target, the disintegration of the nanocomposite was induced through target hybridization and DNA hydrolyzation, leading to the separation of AuNCs and R-CDs NS, and the ET disappeared and led to a high photocurrent. With duplex-specific nuclease enzyme-assisted target recycling, the high sensitivity enabled the sensor to monitor the target in cancer cells. The sensor has a low detection limit of 71 aM. The sensing platform has high sensitivity, good selectivity, and reproducibility.

A photoelectrochemical biosensor based on SnO2 nanoparticles for phosphatidylcholine detection in soybean oil

A photoelectrochemical (PEC) biosensor based on SnO₂ nanoparticles (SnO₂ NPs) was developed and applied for phosphatidylcholine (PC) detection in soybean oil. SnO₂ NPs were grown on an indium tin oxide (ITO) electrode, polythionine (PTh) was electropolymerized on the surface of ITO/SnO₂ NPs, and choline oxidase (ChO_x) was immobilized to prepare the ITO/SnO₂ NPs/PTh/ChO_x electrode. The developed PEC biosensor can detect PC under visible light irradiation. The experimental conditions for PC detection were as follows: 1.8 mg mL^-1 ChO_x concentration, 0.5 V bias voltage, 18 mW cm^-2 light intensity, and pH 6. The PEC biosensor had a detection limit of 0.005 mM (S/N = 3) and a detection range from 0.03 mM to 4 mM. This PEC biosensor based on SnO₂ NPs was applied to detect PC in soybean oil. The recovery rate tested by the standard addition method was 95.2-107.4%. These findings were consistent with the results obtained by high-performance liquid chromatography (HPLC). Therefore, the proposed PEC biosensor based on SnO₂ NPs has excellent reproducibility, stability, and great potential applications in the PEC analysis of PC in soybean oil.

An enhanced photoelectrochemical biosensor for colitoxin DNA based on HKUST-1/TiO2 and derived HKUST-CuO/TiO2 heterogeneous composites

HKUST-1 MOFs and its derivative HKUST-CuO were coupled with TiO₂ nanoparticles to form the heterogeneous composites of HKUST-1/TiO₂ and HKUST-CuO/TiO₂ based on their well-suitable bandgap energies (Eg). Compared with mono-component HKUST-1 or HKUST-CuO, the prepared composites displayed photoelectrochemical (PEC) response due to the synergistic effect from their heterogeneous structure. Higher photocurrent response was obtained on HKUST-CuO/TiO₂-modified ITO electrode (HKUST-CuO/TiO₂/ITO), which could be attributed to the hollow structure with a thin shell of HKUST-CuO greatly enhancing visible spectra harvesting. The CuO component in HKUST-CuO not only could accelerate electron transfer on the heterojunction interface but also effectively separate the photo-generated charge carriers (e^-1/h⁺). Based on the excellent PEC performance of prepared photoactive composite material, under visible-light excitation (λ ≥ 420 nm) and with a working potential of 0 V (vs. Ag/AgCl), the S1 (probe DNA)/HKUST-CuO/TiO₂/ITO PEC platform was successfully fabricated for colitoxin DNA detection without using ascorbic acid (AA) as an electron donor. Compared with the analysis results on S1/HKUST-1/TiO₂/ITO electrode, S1/HKUST-CuO/TiO₂/ITO displayed a wider linear response range from 1.0 × 10^-6 to 4.0 × 10^-1 nM with a lower detection limit of 3.73 × 10^-7 nM (S/N = 3), the linear regression equation was ΔI (10^-6 A) =0.5549-0.1858 log (C_S2/M), which confirmed the HKUST-CuO could improve sensitivity because of its prominent PEC property. The relative standard deviation (RSD) of the PEC sensor for target DNA detection of 2.0 × 10^-4 nM was 7.4%. The proposed DNA biosensor also possessed good specificity and stability. Hence, this reported work was a promising strategy for molecular diagnosis in the bio-analysis field. (A) Schematic illustration of the preparation process of the proposed PEC biosensors for colitoxin DNA detection. (B) The preparation process of HKUST-1 and HKUST-CuO.

Dual-Emission Ratiometric Fluorescent Probe Based on Lanthanide-Functionalized Carbon Quantum Dots for White Light Emission and Chemical Sensing

Herein, we develop a novel method to synthesize lanthanide-functionalized carbon quantum dots via free-radical copolymerization using the methyl methacrylate (MMA) monomer as a functional monomer and introducing a lanthanide complex to obtain the dual-emission fluorescent composite material FCQDs-Ln(TFA)₃ (Ln = Eu, Tb; TFA: trifluoroacetylacetone). The obtained composites were fully characterized, and their structures were investigated by Fourier transform infrared spectroscopy (FTIR), ¹H NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Subsequently, a series of white-light-emitting polymer composite films FCQDs- (Eu:Tb)(TFA)₃/poly(methyl methacrylate) (PMMA) were designed and synthesized by adjusting the ratio of Eu(TFA)₃/Tb(TFA)₃ under different wavelengths. More significantly, FCQDs-Tb(TFA)₃ was selected as a sensitive probe for sensing metal cations due to excellent photoluminescence properties, revealing a unique capability of FCQDs-Tb(TFA)₃ of detecting Fe(III) cations with high efficiency and selectivity. Furthermore, the sensing experiment results indicated that FCQDs-Tb(TFA)₃ is ideal as a fluorescent nanoprobe for Fe³⁺ ion detection, and the lowest detection limit for Fe³⁺ is 0.158 μM, which is superior to many other previous related research studies. This pioneering work provides a new idea and method for constructing a dual-emission ratio sensor based on carbon quantum dots and also extends the potential application in the biological and environmental fields.

Label-Free Antifouling Photoelectrochemical Sensing Strategy for Detecting Breast Tumor Cells Based on Ligand-Receptor Interactions

Biomarker expression both on the cell surface and in serum is directly related to the pathological process of tumor. Based on the interaction between the ligand and the protein receptor, a label-free photoelectrochemical (PEC) biosensing interface with good antifouling ability was proposed for tumor cell detection. TiO₂ nanotube (NT) arrays were used as the substrate to enhance the ability of the biosensor to capture the target. Mercapto-terminated 8-arm poly(ethylene glycol) was introduced onto the electrode surface by the deposition of Au nanoparticles on TiO₂ NTs, creating an antifouling molecular layer. The recognition ligand hyaluronic acid (HA) was functionalized by dopamine and introduced onto the sensing surface based on the unique chelating interaction between the catechol group and the titanium atom. Benefitting from the specific recognition of HA with CD44 and the 3D porous structures of NTs, the constructed PEC biosensor showed excellent abilities toward the detection of MDA-MB-231 breast tumor cells and the soluble form of CD44. The ligand-receptor PEC sensing strategy has promising potential for the detection of tumor cells and protein biomarkers.

Label-Free and Sensitive Determination of Cadmium Ions Using a Ti-Modified Co3O4-Based Electrochemical Aptasensor

The current work demonstrates an electrochemical aptasensor for sensitive determination of Cd²⁺ based on the Ti-modified Co₃O₄ nanoparticles. In this unlabeled system, Ti-modified Co₃O₄ nanoparticles act as current signal amplifiers modified on the screen-printed carbon electrode (SPCE) surface, while the derivative aptamer of Cd²⁺ works as a target recognizer. In addition, the sensing is based on the increase in electrochemical probe thionine current signal due to the binding of aptamer to Cd²⁺ via specific recognition. In the current study, key parameters, including aptamer concentration, pH, and incubation time were optimized, respectively, to ensure sensing performance. Cyclic voltammetry was used not only to characterize each preparation and optimization step, but also to profile the bindings of aptamer to Cd²⁺. Under optimal conditions, Cd²⁺ can be determined in a linear range of 0.20 to 15 ng/mL, with a detection limit of 0.49 ng/mL, significantly below the maximum concentration limit set by the U.S. Environmental Protection Agency. Based on comparative analysis and the results of recovery test with real samples, this simple, label-free but highly selective method has considerable potential and thus can be used as an in-situ environmental monitoring platform for Cd²⁺ testing.

Nanobiosensing with graphene and carbon quantum dots: Recent advances

Graphene and carbon quantum dots (GQDs and CQDs) are relatively new nanomaterials that have demonstrated impact in multiple different fields thanks to their unique quantum properties and excellent biocompatibility. Biosensing, analyte detection and monitoring wherein a key feature is coupled molecular recognition and signal transduction, is one such field that is being greatly advanced by the use of GQDs and CQDs. In this review, recent progress on the development of biotransducers and biosensors enabled by the creative use of GQDs and CQDs is reviewed, with special emphasis on how these materials specifically interface with biomolecules to improve overall analyte detection. This review also introduces nano-enabled biotransducers and different biosensing configurations and strategies, as well as highlights key properties of GQDs and CQDs that are pertinent to functional biotransducer design. Following relevant introductory material, the literature is surveyed with emphasis on work performed over the last 5 years. General comments and suggestions to advance the direction and potential of the field are included throughout the review. The strategic purpose is to inspire and guide future investigations into biosensor design for quality and safety, as well as serve as a primer for developing GQD- and CQD-based biosensors.

Photoelectrochemical aptasensor for thrombin based on Au-rGO-CuS as signal amplification elements

A photoelectrochemical platform for thrombin determination was developed based on Au-rGO-CuS as multiple signal amplification elements. CuInS₂ QDs was used to sensitize burr-shape TiO₂ (b-TiO₂) to obtain a strong photocurrent. Under the specific recognition between aptamer and thrombin, a sandwichlike structure was formed and the Au-rGO-CuS-labeled aptamer (S2@Au-rGO-CuS) was immobilized on the electrode surface. This induced a sharp decrease in photocurrent. The phenomenon is mainly due to the fact that CuS NPs can competitively consume the light energy and electron donor with CuInS₂/b-TiO₂. The rGO can increase the amount of CuS NPs and the Au NPs can accelerate charge transferring which depress the recombination of photogenerated electrons and holes in CuS to further enhance the competitive capacity of CuS. The sandwichlike structure has a steric hindrance effect. Therefore, the S2@Au-rGO-CuS has a multiple signal amplification function for thrombin determination. Under optimal conditions, the PEC aptasensor exhibited a wide linear concentration range from 0.1 pM to 10 nM with a low detection limit of 30 fM (S/N = 3) for thrombin. Besides, the designed aptasensor performed well in the assay of human serum sample, indicating good potential for the determination of thrombin in real samples. Graphical abstract.

Photoelectrochemical assay for DNA hydroxymethylation determination based on the inhibited photoactivity of black TiO2 nanosphere by ZnO

A photoelectrochemical method was proposed for DNA hydroxymethylation determination using black TiO₂ (B-TiO₂) nanosphere as photoactive material and ZnO as photoactivity inhibitor. After hydroxymethylated DNA (5hmC-DNA) was captured on the probe modified B-TiO₂/ITO electrode surface through hybridization, a glycosyl can be then transferred from uridine diphosphoglucose to 5hmC-DNA and formed a covalent structure with -CH₂OH in the presence of T4 β-glucosyltransferase (β-GT). Afterwards, based on a series of covalent reaction, amino functionalized ZnO nanoparticles are further immobilized to the surface of the electrode. Due to the capacity to expend the irradiation light and the photogenerated electron of electron donor, the modified ZnO nanoparticles can result in a decreased photocurrent. The developed method shows wide linear ranges from 0.05-200 nM for hydroxymethylated DNA and 1-220 unit·mL^-1 for T4-β-glucosyltransferase. The corresponding determination limits were 0.013 nM and 0.24 unit·mL^-1, respectively. The enzyme activity inhibited by 4-phenylimidazole was evaluated. This photoelectrochemical method shows high specificity for 5hmC-DNA (compared to 5fC, 5mC, m6A, control) and β-GT (compared to β-AGT, UGT2B7), and shows excellent stability for testing 5hmC (RSD = 2.75%). Graphical abstractSchematic representation of photoelectrochemical method for DNA hydroxymethylation and β-glucosyltransferase detection based on the glycosylation reaction of -CH₂OH in 5-hydroxymethylcytosine and the inhibition activity of ZnO to the photoactivity of black TiO₂ nanospheres.

Carbon dots-sensitized amorphous MoSx photoanode: Sequential electrodeposition preparation and dual amplified photoelectrochemical aptasensing of adenosine

The design and fabrication of high visible-light activated photoelectrode are essential to precisely detect biomolecule in biological system. Herein, an ultrasensitive photoelectrochemical (PEC) aptasensor for specific recognition of adenosine is established based on carbon dots sensitized-amorphous molybdenum sulfide (a-MoS_x/CDs) photoanode and dual amplification strategy. The heterostructured photoanode achieved by sequential electrodeposition reveals significantly boosted photocurrent with good stability and repeatability under visible light illumination, giving the credit to highly activated visible light absorption, uniform coverage and good electric contact to the underlying substrate, as well as the energy-band alignment between the two components. By stepwisely immobilizing complementary DNA probe (NH₂-DNA) and adenosine aptamer (Apt), followed by methylene blue (MB) binding with the guanine base on Apt, a dual amplified self-powered PEC aptasensor for adenosine detection is constructed. Based on the co-sensitization effect of CDs and MB, ultrasensitive and high-affinitive determination of adenosine is realized over the concentration range of 0.01 nM-1000 nM at 0 V (vs. SCE), with satisfactory stability and reproducibility. The detection limit is as low as 3.3 pM, demonstrating a performance even surpassing most of the sensors reported so far. The prospective application of the co-sensitized a-MoS_x photoanode for ultrasensitive aptasensing is highlighted in this work.

Photoelectrochemical determination of malathion by using CuO modified with a metal-organic framework of type Cu-BTC

A photoelectrochemical (PEC) sensor was constructed for the detection of non-electroactive malathion. It is based on the use of a hierarchical CuO material derived from a Cu-BTC metal-organic framework (where BTC stands for benzene-1,3,5-tricarboxylic acid). The modified CuO was obtained by calcination of Cu-BTC at a high temperature (300 °C) and possesses a high photocurrent conversion efficiency. Under irradiation with visible light and in the presence of malathion, the formation of the CuO-malathion complex on the CuO gave rise to an increase in steric hindrance. This results in a decrease in photocurrent. This novel PEC detection method has a lower detection limit of 8.6 × 10^-11 mol L^-1 and a wide linear range (1.0 × 10^-10 ~ 1.0 × 10^-5 mol L^-1). Graphical abstract Schematic presentation of the Cu-BTC MOF derived photoelectrochemical sensor for non-electroactive malathion detection.

Carbon Dots-Decorated Bi2WO6 in an Inverse Opal Film as a Photoanode for Photoelectrochemical Solar Energy Conversion under Visible-Light Irradiation

This work focuses on the crystal size dependence of photoactive materials and light absorption enhancement of the addition of carbon dots (CDs). mac-FTO (macroporous fluorine-doped tin oxide) films with an inverse opal structure are exploited to supply enhanced load sites and to induce morphology control for the embedded photoactive materials. The Bi₂WO₆@mac-FTO photoelectrode is prepared directly inside a mac-FTO film using a simple in situ synthesis method, and the application of CDs to the Bi₂WO₆@mac-FTO is achieved through an impregnation assembly for the manipulation of light absorption. The surface morphology, chemical composition, light absorption characteristics and photocurrent density of the photoelectrode are analyzed in detail by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis diffuse reflectance spectra (DRS), Energy dispersive X-ray analysis (EDX) and linear sweep voltammetry (LSV).

Photoelectrochemical aptamer-based sensing of the vascular endothelial growth factor by adjusting the light harvesting efficiency of g-C3N4 via porous carbon spheres

A "signal-off" sensor is described for sensitive photoelectrochemical (PEC) determination of the vascular endothelial growth factor (VEGF₁₆₅). Graphitic carbon nitride (g-C₃N₄) is used as the signalling material, and porous carbon spheres as efficient quenchers of the photocurrent. The quenching efficiency of carbon spheres is the result of two effects, viz. (a) the competitive light absorption and (b) competitive electron donor activity which decreases the number of light-generated electrons and holes and also reduces the charge separation efficiency. This new mechanism differs from the previous quenching mechanisms which usually are based on the suppression of electron transport or steric hindrance. A glassy carbon electrode was modified with an aptamer against VEGF₁₆₅. On binding of analyte (VEGF₁₆₅), the reduction of current is measured (at a typical potential of 0 V) using H₂O₂ as the electrochemical probe. The sensor has a linear response in the 10^-5 nM to 10² nM VEGF₁₆₅ concentration range, and the detection limit is 3 fM. Graphical abstract Schematic presentation of the quenching mechanism of carbon spheres: the competitive light absorption and competitive electron donor reduce the number of light-generated electrons in the conduction band (CB) and holes in the valence band (VB) and also reduce the charge separation efficiency.

Amplified photoelectrochemical immunoassay for the tumor marker carbohydrate antigen 724 based on dye sensitization of the semiconductor composite C3N4-MoS2

The authors describe an amplified photoelectrochemical immunoassay for the tumor marker carbohydrate antigen 724 (CA724). The method employs a C₃N₄-MoS₂ semiconductor as the photoelectric conversion layer. The nanocomposite was characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray powder diffraction, and UV-vis diffuse reflectometry. The dye eosin Y was encapsulated into CaCO₃ nanospheres which then were used as labels for antibody against CA724. In addition, Fe₃O₄ nanospheres were employed as magnetic platform for constructing photoelectrochemical sandwich immunoassay. The CaCO₃ nanospheres can be dissolved with aid of ethylene diamine tetraacetic acid (EDTA) and the carried eosin Y in CaCO₃ is released. The released dyes sensitizes the C₃N₄-MoS₂ semiconductor, which induces photocurrent amplification. Under optimal conditions and at a typical working voltage of 0 V (vs. SCE), the photocurrent increases linearly in the range of 0.05 mU mL^-1 to 500 mU mL^-1 of CA724, with a 0.02 mU mL^-1 detection limit. Graphical abstract The C₃N₄-MoS₂ complex, with high efficiency of electron transport, was synthesized to construct a photoelectrochemical analytical platform. A sandwich-type immunoassay was established on the surface of magnetic beads. Carbohydrate antigen 724 in sample was detected sensitively by using sensitization of released eosin Y as signal amplifiery.

Tungsten disulfide (WS2) nanosheet-based photoelectrochemical aptasensing of chloramphenicol

A method is described for photoelectrochemical determination of chloramphenicol (CLOA). It is based on the use of (a) aptamers protected with photoactive WS₂ nanosheets, and (b) DNase I-assisted target recycling. The DNA aptamer without label was employed for recognition of CLOA. In the absence of CLOA, the aptamer is adsorbed on the surface of WS₂. This leads to a decrease of photocurrent due to the steric-hindrance effect of aptamer DNA. The adsorption of WS₂ also protects the aptamer from digestion by DNase. In the presence of CLOA, the aptamer will be desorbed from the WS₂ surface due to formation of an aptamer/CLOA conjugate. This results in an increased photocurrent due to a decreased amount of aptamer DNA on the electrode surface. The increase of photocurrent can be further improved by applying DNase triggered catalytic recycling of CLOA. Under optimal experimental conditions, the response is linear 10 pM - 10 nM CLOA concentration range, with a 3.6 pM lower detection limit (at 3σ). This method is acceptably selective, accurate and stable. It was applied to the determination of CLOA in spiked milk samples and gave satisfactory results. Graphical abstract A simple and sensitive photoelectrochemical apta-biosensor was fabricated for chloramphenicol detection. In this work, WS₂ nanosheets were employed as photoactive material, and DNase I catalytic chloramphenicol recycling strategy was adopted to amplify the detection signal.

Electrochemiluminescent aptasensor for thrombin using nitrogen-doped graphene quantum dots

An electrochemiluminescent (ECL) aptamer based method is described for the determination of thrombin. Three-dimensional nitrogen-doped graphene oxide (3D-NGO) was placed on a glassy carbon electrode (GCE) to provide an electrode surface that displays excellent electrical conductivity and acts as a strong emitter of ECL. The modified electrode was further coated with chitosan via electrodeposition. Finally, the amino-modified aptamer was immobilized on the modified GCE. The interaction between thrombin and aptamer results in a decrease in ECL. The assay has a linear response in the 1 fM to 1 nM thrombin concentration range and a 0.25 fM lower detection limit (at an S/N ratio of 3). The method was applied to the determination of thrombin in spiked human plasma samples, and recoveries ranged between 94 and 105% (with RSDs of <3.6%). The calibration plot was recorded at potential and wavelength of fluorescence emission (wavelength: 445 nm; potential: 0 to -2 V). Graphical abstract A bare glassy carbon electrode (GCE) does not display electrochemiluminescence (ECL). If, however, nitrogen-doped graphene quantum dots, chitosan, and three-dimensional nitrogen-doped graphene oxide (NGQD-chitosan/3D-NGO) are electrodeposited on the GCE, strong ECL can be observed. The ECL intensity decreased after aptamer and bovine serum albumin (BSA) were dropped onto the electrode (curve a). However, the ECL further decreases after addition of thrombin (TB; curve b).