Embedded Au Nanoparticles-Based Ratiometric Electrochemical Sensing Strategy for Sensitive and Reliable Detection of Copper Ions

Surface Plasmon Resonance-Enhanced CdS/FTO Heterojunction for Cu2+ Detection

Copper ion (Cu2+) pollution poses a serious threat to marine ecology and fisheries. However, the complexity of seawater and its interference factors make the online detection of Cu2+ quite challenging. To address this issue, we introduce the concept of the photo-assisted adjustment barrier effect into electrochemical detection, using it as a driving force to generate electrochemical responses. The Schottky barrier demonstrates a remarkable regulatory influence on the electrochemical response under photoexcitation, facilitating the response through Cu2+ adsorption. We developed a 4-MBA-AuNPs/CdS/FTO composite that serves as a sensitive platform for Cu2+ detection, achieving a detection limit of 70 nM. Notably, the photo-assisted adjustment of the barrier effect effectively counters the interference posed by ions in seawater, ensuring accurate detection. Furthermore, the sensor exhibits a promising recovery rate (99.62-104.9%) in real seawater samples, highlighting its practical applications. This innovative approach utilizing the photo-assisted adjustment barrier effect offers a promising path for developing electrochemical sensors that can withstand interference.

A sensitive and versatile electrochemical sensor based on hybridization chain reaction and CRISPR/Cas12a system for antibiotic detection

A sensitive electrochemical platform was constructed with NH2-Cu-MOF as electrochemical probe to detect antibiotics using CRISPR/Cas12a system triggered by hybridization chain reaction (HCR). The sensing system consists of two HCR systems. HCR1 occurred on the electrode surface independent of the target, generating long dsDNA to connect signal probes and producing a strong electrochemical signal. HCR2 was triggered by target, and the resulting dsDNA products activated the CRISPR/Cas12a, thereby resulting in effective and rapid cleavage of the trigger of HCR1, hindering the occurrence of HCR1, and reducing the number of NH2-Cu-MOF on the electrode surface. Eventually, significant signal change depended on the target was obtained. On this basis and with the help of the programmability of DNA, kanamycin and ampicillin were sensitively detected with detection limits of 60 fM and 10 fM (S/N = 3), respectively. Furthermore, the sensing platform showed good detection performance in milk and livestock wastewater samples, demonstrating its great application prospects in the detection of antibiotics in food and environmental water samples.

A Novel CRISPR/Cas12a-Mediated Ratiometric Dual-Signal Electrochemical Biosensor for Ultrasensitive and Reliable Detection of Circulating Tumor Deoxyribonucleic Acid

Circulating tumor DNA (ctDNA) holds great promise as a noninvasive biomarker for cancer diagnosis, treatment, and prognosis. However, the accurate and specific quantification of low-abundance ctDNA in serum remains a significant challenge. This study introduced, for the first time, a novel exponential amplification reaction (EXPAR)-assisted CRISPR/Cas12a-mediated ratiometric dual-signal electrochemical biosensor for ultrasensitive and reliable detection of ctDNA. To implement the dual-signal strategy, a signal unit (ssDNA-MB@Fc/UiO-66-NH2) was prepared, consisting of methylene blue-modified ssDNA as the biogate to encapsulate ferrocene signal molecules within UiO-66-NH2 nanocarriers. The presence of target ctDNA KRAS triggered EXPAR amplification, generating numerous activators for Cas12a activation, resulting in the cleavage of ssDNA-P fully complementary to the ssDNA-MB biogate. Due to the inability to form a rigid structure dsDNA (ssDNA-MB/ssDNA-P), the separation of ssDNA-MB biogate from the UiO-66-NH2 surface was hindered by electrostatic interactions. Consequently, the supernatant collected after centrifugation exhibited either no or only a weak presence of Fc and MB signal molecules. Conversely, in the absence of the target ctDNA, the ssDNA-MB biogate was open, leading to the leakage of Fc signal molecules. This clever ratiometric strategy with Cas12a as the "connector", reflecting the concentration of ctDNA KRAS based on the ratio of the current intensities of the two electroactive signal molecules, enhanced detection sensitivity by at least 60-300 times compared to single-signal strategies. Moreover, this strategy demonstrated satisfactory performance in ctDNA detection in complex human serum, highlighting its potential for cancer diagnosis.

Photoelectrochemical aptasensing with methylene blue filled Ni-MOFs nanocomposite by spatial confinement for microcystin-LR detection

Microcystin LR (MC-LR) is a hazardous cyanotoxin produced by cyanobacteria during freshwater eutrophication, which can cause liver cancer. Here, a photoelectrochemical (PEC) aptasensor based on methylene blue (MB)-loaded Ni-MOF composite (Ni-MOF/MB) with spatial confinement was constructed for the sensitive detection of MC-LR. Ni-MOF with two-dimensional sheet structure was prepared via a liquid-liquid interface synthesis method with environmental-friendly solvent and milder reaction conditions. Benefiting from the uniform pore size, Ni-MOF acted as reaction platform to anchor the photosensitive molecule MB. The electron donor, ascorbic acid (AA), was produced by alkaline phosphatase (ALP) loaded on DNA strand catalyzing ascorbic acid phosphate. The generated AA was absorbed by Ni-MOF/MB, thereby effectively improving the utilization of AA and avoiding the external environment interferences to enlarge the photocurrent of MB. For analysis, ALP-labeled aptamer can specifically recognize MC-LR by forming a complex to strip from aptasensor, thus leading to a  decreased photocurrent. The developed PEC aptasensor offered a linear range of 10 fM-100 pM with a detection limit of 6 fM. It was successfully employed for detecting MC-LR in farm water and fish meat, and the results were validated by ultrahigh-performance liquid chromatography-mass spectrometry. This method presents a new idea of MOF-limited domain for PEC aptasensing.

A ratiometric electrochemical sensing strategy based on the self-assembly of Co NC/CNT and methylene blue for effective detection of the food additive tert-butylhydroquinone

It is crucial to achieve accurate and rapid detection of tert-butylhydroquinone (TBHQ) in the field of food safety, for the excessive addition of TBHQ in food is harmful to human health and evil to the environment and aquatic life. Therefore, researchers have done a lot of work on signal amplification through nanomaterials to achieve TBHQ detection, but the conventional single-signal detection strategy results in limited accuracy. In this work, an innovative and facile ratiometric electrochemical sensor for TBHQ detection was built based on advanced nanomaterial complexes carbon nanotube-encapsulated Co/nitrogen-doped carbon (Co NC/CNT) and selected internal reference signal methylene blue (MB) enhancing the accuracy by offering effective self-calibration. A linear relationship between the net peak current ratio between TBHQ and MB (ΔI (TBHQ)/ΔI (MB)) and the TBHQ concentration was obtained under the optimal experimental conditions, with two linear ranges of 0.1-20 μM and 20-100 μM and a limit of detection (LOD) of 0.054 μM (S/N = 3). Benefiting from the synergistic effects between Co NC and CNT and the ratiometric sensing strategy, the as-designed sensor for TBHQ detection showcased excellent selectivity, repeatability, reproducibility, stability, and satisfactory applicability in real edible oil samples.

Smartphone-based wearable microfluidic electrochemical sensor for on-site monitoring of copper ions in sweat without external driving

The directional movement of liquid without exogenous drive can show great potential in portable electrochemical platforms. Herein, we developed a portable electrochemical platform that drove electrolyte flow by surface tension gradient, which can realize collection of electrolyte, flow preconcentration and electrochemical detection of Cu2+. The induced graphene electrodes (LIG) was fabricated using laser direct writing, and flower cluster shaped ZnO nanorods (FC-ZnONRs) were prepared and modified on LIG, which provided a large amount of space for electrolyte to shuttled between the holes of LIG and ZnO, and increased the electrochemical active sites and electrons transport ability. The effect of surface tension gradients driving fluid flow could accelerate preconcentration, shorten detection time (save 300 s of preconcentration time) and enhance electrochemical responses in synergy with the 3D FC-ZnONRs/LIG. The microfluidic system possessed excellent performance for detection of Cu2+ ranged from 1 μg L-1 to 2100 μg L-1 with a low detection limit (LOD) of 0.0368 μg L-1 and high sensitivity of 0.414 μA (μg L-1)-1 cm-2. Additionally, this portable microfluidic system was successfully worn on the skin for analysing Cu2+ in human sweat, and the results showed good consistency with inductively coupled plasma-mass spectrometry (ICP-MS). This novel sensing system provides a sample collection, rapid detection, low cost and easy-to-operate strategy for heavy metal ions analysis in real samples and shows huge application prospects in point-of-care testing.

Rapid detection of foodborne pathogens in diverse foodstuffs by universal electrochemical aptasensor based on UiO-66 and methylene blue composites

Rapid and sensitive detection of foodborne pathogens in complex environments is essential for food protection. A universal electrochemical aptasensor was fabricated for the detection of three common foodborne pathogens, including Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and Salmonella typhimurium (S. typhimurium). The aptasensor was developed based on the homogeneous and membrane filtration strategy. Zirconium-based metal-organic framework (UiO-66)/methylene blue (MB)/aptamer composite was designed as a signal amplification and recognition probe. Bacteria were quantitatively detected by the current changes of MB. By simply changing the aptamer, different bacteria could be detected. The detection limits of E. coli, S. aureus and S. typhimurium were 5, 4 and 3 CFU·mL^-1, respectively. In humidity and salt environments, the stability of the aptasensor was satisfactory. The aptasensor exhibited satisfactory detection performance in different real samples. This aptasensor has excellent potential for rapid detection of foodborne pathogens in complex environments.

Amino-Functionalized Hierarchical Porous Carbon Derived from Zeolitic Imidazolate Frameworks for Ultrasensitive Electrochemical Sensing of Heavy Metals in Water

Electrochemical sensing provides a feasible avenue to monitor heavy metal ions (HMIs) in water, whereas the construction of highly sensitive and selective sensors remains challenging. Herein, we fabricated a novel amino-functionalized hierarchical porous carbon by the template-engaged method using ZIF-8 as the precursor and polystyrene sphere as the template, followed by carbonization and controllable chemical grafting of amino groups for efficient electrochemical detection of HMIs in water. The amino-functionalized hierarchical porous carbon features an ultrathin carbon framework with a high graphitization degree, excellent conductivity, unique macro-, meso-, and microporous architecture, and rich amino groups. As a result, the sensor exhibits prominent electrochemical performance with significantly low limits of detection for individual HMIs (i.e., 0.93 nM for Pb²⁺, 2.9 nM for Cu²⁺, and 1.2 nM for Hg²⁺) and simultaneous detection of HMIs (i.e., 0.62 nM for Pb²⁺, 1.8 nM for Cu²⁺, and 0.85 nM for Hg²⁺), which are superior to most reported sensors in the literature. Moreover, the sensor displays excellent anti-interference ability, repeatability, and stability for HMI detection in actual water samples.

Ti3C2@Bi2O3 nanoaccordion for electrochemical determination of miRNA-21

Based on a dual signal amplification strategy of novel accordion-like Bi₂O₃-decorated Ti₃C₂ (Ti₃C₂@Bi₂O₃) nanocomposites and hybridization chain reaction (HCR), an ultra-sensitive electrochemical biosensor was constructed for miRNA-21 detection. By etching Ti₃AlC₂ with HF, Ti₃C₂ with an accordion-like structure was first obtained and subsequently covered by Bi₂O₃ nanoparticles (NPs), forming Ti₃C₂@Bi₂O₃. A layer of Au NPs was electrodeposited on the glassy carbon electrode coated with Ti₃C₂@Bi₂O₃, which not only significantly improved the electron transport capacity of the electrode but also greatly increased its surface active area. Upon the immobilization of the thiolated capture probe (SH-CP) on the electrode, the target miRNA-21 specifically hybridized with SH-CP and thus opened its hairpin structure, triggering HCR to form a long double strand with the primers H1 and H2. A large number of the electrochemical indicator molecules were thus embedded inside the long double strands to produce the desirable electrochemical signal at a potential of - 0.19 V (vs. Ag/AgCl). Such dual signal amplification strategy successfully endowed the biosensor with ultra-high sensitivity for miRNA-21 detection in a wide linear range from 1 fM to 100 pM with a detection limit as low as 0.16 fM. The excellent detection of miRNA-21 in human blood plasma displayed a broad prospect in clinical diagnosis. An ultra-sensitive electrochemical biosensor was successfully constructed for miRNA-21 detection in human blood plasma based on the dual signal amplification strategy of novel accordion-like Bi₂O₃ decorated Ti₃C₂ (Ti₃C₂@Bi₂O₃) nanocomposites and hybridization chain reaction.

PbS/graphene hybrid nanostructures coated glassy carbon electrode for the electrochemical sensing of copper ions in aqueous solution

In this research, we have studied the electrochemical sensing of Cu(II) ions in aqueous solution using PbS/Graphene composite nanostructure coated glassy carbon electrode (GCE). The SEM-EDAX analysis revealed that the lead sulphide nanocrystals are homogeneously embedded on the graphene nanosheets with an uniform particle size of 100 nm, and the elements presents 92.32% and Lead content of 5.45% and Sulfur content of 0.91%. Raman spectra exhibits G with respect to the E2g sp² hybridized C-C and D band with respect to the A1g mode in the disordered edge region of the GNS. The composite nanostructure coated GCE (PbS/Graphene/GCE) was prepared and its performance in the existence of metal ions such us Cd(II),Pb(II), Mg(II), Cu(II) and Ni(II) was studied using the current voltage curves in double distilled water within the scan rates of 25 to 300mVs-¹. The PbS/Graphene coated carbon electrode exhibited the higher anodic and cathodic peak current for copper solution than the other metal ions studied, which various linearly proportional to concentration. The electrochemical sensing characteristics PbS/GNS/GCE was found to be significant towards detecting Cu²⁺ ion within the concentration range of 1 × 10^-4 to 1 × 10^-8 M, with a lowest sensing detection limit of 1 × 10^-8 M.

One-pot synthesis of copper nanocluster/Tb-MOF composites for the ratiometric fluorescence detection of Cu2

The increasing degradation of ecosystems due to heavy metal residues has led to environment and food contamination, prompting the development of convenient platforms for monitoring heavy metals. Here, a new dual-emission fluorescent sensor CuNCs@Tb@UiO-66-(COOH)₂ for the detection of copper ions (Cu²⁺ ) has been synthesized by one-pot encapsulation of Tb (III) and glutathione-stabilized copper nanoclusters (CuNCs) into metal-organic frameworks (MOFs) UiO-66-(COOH)₂ . In this ratiometric sensor, the fluorescence intensity of Tb³⁺ decreased significantly upon the addition of Cu²⁺ , while that of CuNCs showed good stability, together with an apparent color change. Therefore, ratiometric fluorescence detection of Cu²⁺ can be accomplished by measuring the ratio of the fluorescence intensity at the 450 nm (F₄₅₀ ) wavelength of CuNCs to the 548 nm (F₅₄₈ ) emission of Tb³⁺ in the fluorescence spectra of the CuNCs@Tb@UiO-66-(COOH)₂ suspension. Moreover, the obtained fluorescent probe shows good results in the detection of actual samples. This work can provide the basis of method for the exploration of ratiometric fluorescence and visual sensors of trace pollutants analysis in complicated samples.

Transition metal copper composite ionic liquid self-built ratiometric sensor for the detection of paracetamol

In this paper, a MOF derivative Cu-PF₆-MOF composed of transition metal copper and ionic liquid [BMIM] PF₆ was used to construct a ratiometric electrochemical sensor for paracetamol detection. Cu-PF₆-MOF was synthesized by chemical bath method and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray Photoelectron Spectroscopy (XPS) and X-ray powder diffraction (XRD). Owing to the introduction of ionic liquid [BMIM] PF₆ and the synthetic effect on transition metal copper and ionic liquid [BMIM] PF₆, Cu-PF₆-MOF has the higher conductivity, larger electroactive surface area and better intrinsic catalytic properties of the skeletal transition metal, exhibiting enhanced electrocatalytic response to the reduction of paracetamol. The stable reduction peak at -104 mV was used as the ratiometric signal for analytical paracetamol detection using differential pulse voltammetry (DPV), with a linear range of 0.1-100 μM and a detection limit of 0.03 μM (S/N = 3). In addition, the constructed sensor showed good reproducibility, stability and interference resistance, as well as ideal recoveries (98.20%-104.40%) for the analysis of paracetamol in water samples.

"Off-On" typed upconversion fluorescence resonance energy transfer probe for the determination of Cu2+ in tap water

Detection of copper plays a prominent role in the environmental protection and human health. Herein, we firstly design and construct an "off-on" upconversion fluorescence resonance energy transfer (UFRET) probe with low toxicity for the Cu²⁺ determination by using NaYF₄: Yb³⁺, Er³⁺ upconversion nanoparticles (UCNPs) and Au NPs. UCNPs with positive charge and Au NPs with negative charge are respectively employed as the donor and acceptor, and bound together to form UFRET probe. The upconversion fluorescence quenching of UCNPs occurs by Au NPs through FRET (defined as "off" state). When Cu²⁺ exists in samples, Cu²⁺ reacts with 4-mercaptobenzoic acid (4-MBA) capped on the surface of Au NPs to make Au NPs detach from UCNPs, leading to the termination of FRET and the recovery of upconversion fluorescence (defined as "on" state). "Off-on" typed UFRET probe has excellent sensing performances, including linear range of 0.02-1 μM Cu²⁺ concentration, the limit of detection of 18.2 nM, high selectivity to Cu²⁺ and good recovery. The probe has been successfully used to determine Cu²⁺ in spiked tap water with satisfactory results. The probe will provide theoretical and technical support for the design of new sensitive heavy metal ion detection probe.

Smartphone-based electrochemical analysis integrated with NFC system for the voltammetric detection of heavy metals using a screen-printed graphene electrode

The electrochemical determination of five heavy metals is demonstrated using a wireless and card-sized potentiostat coupled with a smartphone through near-field communication (NFC) technology. A smartphone application was customized to command the NFC potentiostat, collect real-time signals, process the data, and ultimately display the quantities of the selected elements. The screen-printed graphene electrode (SPGE) was simply fabricated and modified using different nanomaterials for each heavy metal. Using differential pulse voltammetry (DPV) mode on the smartphone, the signal peaks were presented at + 10 mV for As(III), + 350 mV for Cr(VI), 0 mV for Hg(II), - 900 mV for Cd(II), and - 680 mV vs. Ag/AgCl for Pb(II). The linear ranges were 25-500, 250-25,000, 100-1,500, 25-750, 25-750 ng mL^-1 with detection limits of 3.0, 40, 16, 2.0, and 0.95 ng mL^-1 for As(III), Cr(VI), Hg(II), Cd(II), and Pb(II), respectively. The reproducibility in terms of relative standard deviation was less than 8.8% (n = 5 devices) of the developed SPGE coupled with the NFC potentiostat. Various samples for different applications (e.g., food safety and environmental monitoring) were analyzed and quantified using the proposed sensors. The results from this sensor indicate that there is no significant difference (95% confidence level) compared with those obtained from the traditional ICP-OES method, while the recoveries were found in the acceptable range of 80-111%. Hence, it can be deduced that this recent advanced technology of the NFC potentiostat developed for heavy metal analysis offers a highly sensitive and selective detection, yet the sensor remains compact, low-cost, and readily accessible to end-users.

Covalent organic framework modified carbon cloth for ratiometric electrochemical sensing of bisphenol A and S

A novel ratiometric electrochemical sensor was developed based on a carbon cloth electrodeposited with silver nanoparticles and drop-coated by covalent organic framework (COF-LZU1) for simultaneous determination of bisphenol A (BPA) and bisphenol S (BPS). Carbon cloth exhibited a significantly larger electrochemical active area than common glassy carbon electrodes (27.5 times). Silver nanoparticles not only provided a stable reference signal but also enhanced electroactivity for the oxidation of BPA and BPS. COF-LZU1 with good adsorption performance and large periodic π-arrays promoted the enrichment of BPA and BPS to further increase the current response. Compared with the traditional single-signal electrochemical sensor, the developed ratiometric sensor exhibited better reproducibility and a wider linear range for BPA and BPS from 0.5 to 100 μM with a limit of detection of 0.15 μM. Furthermore, the developed sensor showed excellent stability and superior anti-interference ability. The real sample analysis for BPA and BPS has been successfully carried out in mineral water, electrolyte drink, tea, juice, and beer with recoveries of 88.3-111.7%. The developed ratiometric sensor is expected to be a candidate for the preparation of other electrochemical sensors and the analysis of additional practical samples.

Catalytic Hairpin Assembly-Driven Ratiometric Dual-Signal Electrochemical Biosensor for Ultrasensitive Detection of MicroRNA Based on the Ratios of Fe-MOFs and MB-GA-UiO-66-NH2

In this work, a novel ratio electrochemical biosensing platform based on catalytic hairpin assembly target recovery to trigger dual-signal output was developed for ultrasensitive detection of microRNA (miRNA). To achieve the ratiometric dual-signal strategy, methylene blue (MB), an electrochemical indicator, was ingeniously loaded into the pores of graphene aerogel (GA) and metal-organic framework (MOF) composites with high porosity and large specific surface area, and another electrochemical indicator Fe-MOFs with distinct separation of redox potential was selected as a signal probe. Concretely, with the presence of the target miRNA, the CHA process was initiated and the signal probe was introduced to the electrode surface, producing abundant double-stranded H1-H2@Fe-MOFs-NH₂. Then, the measurement and analysis of the prepared ratiometric electrochemical biosensor by differential pulse voltammetry (DPV) showed that the introduction of the target miRNA led to an increase in the oxidation peak signal of Fe-MOFs (+0.8 V) and a decrease in the oxidation peak signal of MB (-0.23 V). Therefore, the peak current ratio of I_Fe-MOFs/I_MB could be employed to accurately reflect the actual concentration of miRNA. Under optimal conditions, the detection limit of the proposed biosensor was down to 50 aM. It was worth noting that the proposed biosensor exhibited excellent detection performance in a complex serum environment and tumor cell lysates, showing great potential in biosensing and clinical diagnosis.

Real time sensor for Fe3+, Al3+, Cu2+ & PPi through quadruple mechanistic pathways using a novel dipodal quinoline-based molecular probe

A quinoline-based small molecular probe, H₂L was designed, synthesized and characterized by different spectroscopic methods. It was utilized as a multi-responsive probe for the detection of Fe³⁺, Al³⁺, Cu²⁺ and PPi. It showed very selective instant turn-on fluorimetric response towards Fe³⁺and Al³⁺ with a detection limit in nanomolar range. Solutions of H₂L containing Fe³⁺ or Al³⁺ could sequentially sense PPi by a turn-off mechanism. Also, H₂L could determine the presence of Cu²⁺ very selectively among a series of other metal ions by a sharp change in colour. Detection of Cu²⁺ through colorimetry was further investigated by systematic UV-Vis studies and the potential of H₂L to act as a potential colorimetric sensor for Cu²⁺ was suitably established. Filter-paper strip experiments were conducted to demonstrate the practical utility of the proposed sensor. Potential applications of H₂L as a sensor for pH in the acidic range has also been explored.

A Simple Colorimetric Assay for Sensitive Cu2+ Detection Based on the Glutathione-Mediated Etching of MnO2 Nanosheets

In this paper, we developed a quick, economical and sensitive colorimetric strategy for copper ions (Cu²⁺) quantification via the redox response of MnO₂ nanosheets with glutathione (GSH). This reaction consumed MnO₂ nanosheets, which acted as a catalyst for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product (oxTMB). In the presence of Cu²⁺, the GSH was catalyzed to GSSG (oxidized glutathione), and the solution changed from colorless to deep blue. Under the optimum conditions, the absorption signal of the oxidized product (oxTMB) became proportional to Cu²⁺ concentration in the range from 10 to 300 nM with a detection limit of 6.9 nM. This detection system showed high specificity for Cu²⁺. Moreover, the system has been efficaciously implemented for Cu²⁺ detection in actual tap water samples. The layered-nanostructures of MnO₂ nanosheets make it possess high chemical and thermal stability. TMB can be quickly oxidized within 10 min by the catalyzing of MnO₂ nanosheets with high oxidase-like activity. There is no need of expensive reagents, additional H₂O₂ and complicated modification processes during the colorimetric assay. Therefore, the strategy primarily based on MnO₂ nanosheets is promising for real-time, rapid and highly sensitive detection of Cu²⁺ under practical conditions.

Graphene Biodevices for Early Disease Diagnosis Based on Biomarker Detection

The early diagnosis of diseases plays a vital role in healthcare and the extension of human life. Graphene-based biosensors have boosted the early diagnosis of diseases by detecting and monitoring related biomarkers, providing a better understanding of various physiological and pathological processes. They have generated tremendous interest, made significant advances, and offered promising application prospects. In this paper, we discuss the background of graphene and biosensors, including the properties and functionalization of graphene and biosensors. Second, the significant technologies adopted by biosensors are discussed, such as field-effect transistors and electrochemical and optical methods. Subsequently, we highlight biosensors for detecting various biomarkers, including ions, small molecules, macromolecules, viruses, bacteria, and living human cells. Finally, the opportunities and challenges of graphene-based biosensors and related broad research interests are discussed.

Highly Sensitive and Stable Determination of As(III) under Near-Neutral Conditions: Benefit from the Synergetic Catalysis of Pt Single Atoms and Active S Atoms over Pt1/MoS2

Designing new catalysts with high activity and stability is crucial for the effective analysis of environmental pollutants under mild conditions. Here, we developed a superior catalyst of Pt single atoms anchored on MoS₂ (Pt₁/MoS₂) to catalyze the determination of As(III). A detection sensitivity of 3.31 μA ppb^-1 was obtained in acetate buffer solution at pH 6.0, which is the highest compared with those obtained by other Pt-based nanomaterials currently reported. Pt₁/MoS₂ exhibited excellent electrochemical stability during the detection process of As(III), even in the coexistence of Cu(II), Pb(II), and Hg(II). X-ray absorption fine structure spectroscopy and theoretical calculations revealed that Pt single atoms were stably fixed by four S atoms and activated the adjacent S atoms. Then, Pt and S atoms synergistically interacted with O and As atoms, respectively, and transferred some electrons to H₃AsO₃, which change the rate-determining step of H₃AsO₃ reduction and reduce reaction energy barriers, thereby promoting rapid and efficient accumulation for As(0). Compared with Pt nanoparticles, the weaker interaction between arsenic species and Pt₁/MoS₂ enabled the effortless regeneration and cyclic utilization of active centers, which is more favorable for the oxidation of As(0). This work provides inspiration for developing highly efficient sensing platforms from the perspective of atomic-level catalysis and affords references to explore the detection mechanism of such contaminants.

AuPt NPs with enhanced electrochemical oxidization activity for ratiometric electrochemical aptasensor

Strong and stable electrochemical beacons are critical for the achievement of sensitive and reliable electroanalysis applications. In this work, the electrochemical oxidation performance of AuPt NPs was studied and firstly found to be largely enhanced under light illumination. Plasmonic AuPt NPs collected light energy after local surface plasmon resonance (LSPR) excitation and generated much more holes to participate in the electrochemical oxidation process of Pt⁰ in AuPt NPs. AuPt NPs with the electrochemical oxidation peak at around -0.7 V were utilized as detection probes for the fabrication of ratiometric electrochemical aptasensor, by introducing Co-MOF/Fe₃O₄/Ag nanosheets (NSs) with the electrochemical oxidation peak at 0.1 V as reference beacons. The aptamers of epithelial cell adhesion molecule (EpCAM) modified AuPt NPs were assembled with Co-MOF/Fe₃O₄/Ag NSs, which generated strong detection and reference signals at -0.7 V and 0.1 V, respectively. The high affinity between EpCAM and aptamers induced the separation of AuPt NPs from Co-MOF/Fe₃O₄/Ag NSs, resulting in the decrease of detection signal at -0.7 V and unchanged reference signal at 0.1 V. A ratiometric electrochemical aptasensor was achieved for the sensitive and reliable quantification of EpCAM in the range from 100 pg/mL to 100 ng/mL. The limit of detection (LOD) was calculated to be 13.8 pg/mL for EpCAM. Plasmon-driven electrochemical oxidation enhancement principle provides the possibility for the design and fabrication of more strong and anti-interference electroactive plasmonic metal-Pt composite nanostructures for the electroanalysis applications.

Photo-assisted simultaneous electrochemical detection of multiple heavy metal ions with a metal-free carbon black anchored graphitic carbon nitride sensor

The simultaneous detection of multiple heavy metal ions in solution is an important yet highly challenging problem. In this work, a metal-free g-C₃N₄/carbon black (CB) composite electrode was synthesized by a one-step thermal polycondensation method and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and ultraviolet visible light spectroscopy. In addition, the photoelectrochemical response of the g-C₃N₄/CB nanocomposite to Cd²⁺, Pb²⁺ and Hg²⁺ both separately and as a mixture of the three analytes was investigated by differential pulse anodic stripping voltammetry. The g-C₃N₄/CB electrode demonstrated an excellent sensing performance to Cd²⁺, Pb²⁺ and Hg²⁺ in the range of 0-700 nM, 0-300 nM and 0-500 nM, respectively, with limits of detection (LOD) of Cd²⁺, Pb²⁺, and Hg²⁺ of 2.1, 0.26 and 0.22 nM, respectively. The LOD of the combined solution of the three analytes was slightly higher at 3.3 nM. Additionally, the metal-free g-C₃N₄/CB photoelectrochemical sensor exhibited excellent electrochemical stability and electrode reproducibility. Finally, g-C₃N₄/CB sensor also showed satisfactory results in the detection of trace analyte ions in real environmental systems. This work provides a novel and promising approach in the simultaneous detection of multiple heavy metal ions in solution for practical applications.

Simultaneous Visual Detection and Removal of Cu2+ with Electrospun Self-Supporting Flexible Amidated Polyacrylonitrile/Branched Polyethyleneimine Nanofiber Membranes

Sensitive detection and effective removal of copper ions (Cu²⁺) from water are still arduous tasks required to protect public health and environmental safety because of the serious impacts of Cu²⁺ on humans and other organisms. Herein, we report the design and fabrication of self-supporting flexible amidated polyacrylonitrile/branched polyethyleneimine nanofiber membranes (abbreviated as aPAN/BPEI NMs) via facile electrospinning and a subsequent hydrothermal method, which are used not only as strips for the visual detection of Cu²⁺ but also as effective adsorbents for the removal of Cu²⁺ from water. Because aPAN/BPEI NMs are self-supporting, they can be easily removed from the solution to reduce secondary pollution to the environment. Based on the high Cu²⁺ binding capacity of BPEI, Cu²⁺ ions are adsorbed on the aPAN/BPEI NMs, which leads to the appearance of new absorbance bands at 280 and 636 nm and a color change from yellow to blue. aPAN/BPEI NMs are utilized for the visual detection of Cu²⁺ with a linear range of 50-700 μM and limits of detection of 11.5 and 4.8 μM (absorption peaks at 280 and 636 nm). More importantly, aPAN/BPEI NMs exhibit excellent selectivity and certain recovery with a simple treatment. Furthermore, by utilizing the adsorption characteristics of Cu²⁺ in aqueous media, it can be effectively removed by aPAN/BPEI NMs with a remarkable adsorption capacity of 209.53 mg·g^-1. Additionally, the removal of Cu²⁺ by aPAN/BPEI NMs does not exhibit interference by other foreign ions. The adsorption process conforms well to the pseudo-second order (PSO) kinetic model and Jovanovich model, proving that adsorption occurs via chemical and monolayer adsorption mechanisms. Accordingly, this work will provide theoretical and technical support for the design and fabrication of novel heavy metal ion detection-removal integrated materials exhibiting high sensitivity and strong adsorption.

Free-electrodeposited anodic stripping voltammetry sensing of Cu(II) based on Ti3C2Tx MXene/carbon black

A proof-of-principle concept for free-electrodeposited anodic stripping voltammetry (ASV) sensing of Cu²⁺ is proposed by using Ti₃C₂T_x MXene/carbon black (Ti₃C₂T_x@CB) nanohybrids as electrode materials. Owing to the high adsorption and reduction capability of Ti₃C₂T_x towards Cu²⁺, Ti₃C₂T_x MXene enables Cu²⁺ to be immobilized and self-reduced directly to form Cu⁰ on the Ti₃C₂T_x@CB electrode surface. As a result an oxidation peak current appears from the re-oxidation of Cu⁰ via differential pulse voltammetry. Carbon black (CB) was introduced to prevent Ti₃C₂T_x Mxene aggregation and improve the related electron transfer as well as enhance their surface area. After optimizing various conditions, a considerable low limit of detection (4.6 nM) and a wide linear range (0.01-15.0 μM) for Cu²⁺ were achieved at the working potential from - 0.3 V to 0.0 V (vs SCE). Relative standard deviation (RSD) of eight individual Ti₃C₂T_x@CB electrodes is 3.72%, and the recoveries from tap water sample and lake water sample were in the ranges of 97.0-108% and 104-107%, respectively.

Determination of Cr(VI) based on the peroxidase mimetic catalytic activity of citrate-capped gold nanoparticles

Highly negatively charged gold nanoparticles (AuNPs) are shown to have strong simulated oxidase activity and effectively boosted the oxidation of enzyme substrate 3,3',5,5'-tetramethylbenzidine (TMB) by hexavalent chromium ion Cr(VI), resulting in the formation of oxidation product with blue color. Based on this, a facile colorimetric assay was developed to detect Cr(VI) at a range 0.008~0.156 mg/L with r = 0.996. The detection limit was estimated to be 0.52 μg/L. In addition, the colorimetric assay showed high selectivity against 28 other interfering ions. It was performed at room temperature and required about half an hour including the preparation of AuNPs. The assay was successfully applied to the determination of Cr(VI) in spiked water samples, and recoveries in the range 95.00-105.40% were obtained. This work paves a way for design of high performance sensor based on highly active nanozymes and also provides an extremely practical analytical tool for the monitoring of Cr(VI) in the environment.

Ultra-sensitive optical fiber sensor based on intermodal interference and temperature calibration for trace detection of copper (II) ions

An ultrahigh sensitive optical fiber sensor for trace detection of Cu²⁺ concentration in aqueous solution with temperature calibration has been developed in this article. Based on the intermodal interference, the sensor is coated with a hydrogel sensing membrane with specific binding to Cu²⁺ on the no-core fiber/single mode fiber/no-core fiber structure by using our new spray coating method. The imidazole group in the sensing film combines with Cu²⁺ to produce chelation, which changes the refractive index of the sensing film. The Cu²⁺ at trace concentration can be detected by monitoring the displacement of the interference trough. The experimental limit of detection of 3.0×10^-12 mol/L can be achieved with the spectral resolution of 0.02 nm. The sensor has also long-term stability of the concentration measurement with the average standard deviation of 1.610×10^-12 mol/L over 2 hours observation time and can be compensated the influence of ambient temperature on concentration detection by conducting the temperature calibration. In addition, the sensor has the advantages of strong specificity, simple fabrication and low cost.

A novel ratiometric electrochemical sensor for the selective detection of citrinin based on molecularly imprinted poly(thionine) on ionic liquid decorated boron and nitrogen co-doped hierarchical porous carbon

Citrinin can cause serious human diseases, thus its detection in foods is necessary. Herein, a molecularly imprinted polymer-based ratiometric electrochemical sensor (MIP-RECS) was presented for citrinin detection. The sensor was fabricated by electropolymerization, using thionine as monomer and citrinin as template. The ionic liquid decorated boron and nitrogen co-doped hierarchical porous carbon (BN-HPC) as supporter, provided large surface for anchoring thionine and citrinin. Poly(thionine) not only acted as MIP, but also acted as reference probe. When [Fe(CN)₆] ^3-/4- was adopted as indicating probe, the resulting sensor demonstrated a wide linear detection range (i.e. 1 × 10^-3-10 ng mL^-1) and a low detection limit (i.e. 1 × 10^-4 ng mL^-1).The sensor was applied to the detection of spiked citrinin in real samples, and satisfactory recovery (i.e. 97% - 110%) was obtained. Hence, it was promising for citrinin detection.

Cobalt encapsulated in bamboo-like N-doped carbon nanotubes for highly sensitive electroanalysis of Pb(II): enhancement based on adsorption and catalysis

Carbon nanotubes (CNTs) are recognized as desirable candidates to fabricate electrochemical sensing interfaces owing to their high surface area and excellent electron conductivity. However, the poor catalytic properties of CNTs significantly hinder their further application in electrochemical detection. Herein, for the first time we combined defective CNTs with catalytically active cobalt nanoparticles (Co NPs) to give cobalt encapsulated in a bamboo-like N-doped carbon nanotube nanocomposite (Co/N-CNTs). The novel constructed Co/N-CNTs are used as a modifier on a bare glass carbon electrode for the electrochemical detection of Pb(ii). As a result, the positive effect of adsorption and catalysis on Co/N-CNT shows a significant improvement in the electroanalytical performance towards Pb(ii) with a sensitivity of 69.74 μA μM-1 and a limit of detection of 0.039 μM. Moreover, the stability and practical applications of Co/N-CNTs towards Pb(ii) in real water samples obtained from a mining subsidence area were also considered. This method shows great promise, achieving an outstanding electroanalysis efficiency with noble-metal-free nanocomposite sensors based on the combination of carbon and transition metals.

MXene@Ag-based ratiometric electrochemical sensing strategy for effective detection of carbendazim in vegetable samples

In this paper, a novel ratiometric electrochemical sensor for carbendazim (CBZ) detection was constructed by a composite of MXene@Ag nanoclusters and amino-functionalized multi-walled carbon nanotubes (MXene@AgNCs/NH₂-MWCNTs). The Ag nanoclusters (AgNCs) embedded in the MXene not only could inhibit the aggregation of MXene flakes and enhance the electrocatalytic ability, but also serve as an internal reference probe for the ratiometric electrochemical detection. Moreover, the introduction of NH₂-MWCNTs can further improve the electrochemical signals of CBZ and Ag, resulting in the enhanced signal amplification and higher sensitivity. Based on these characteristics of the MXene@AgNCs/NH₂-MWCNTs composite, the proposed sensor exhibits a favorable linear relationship between I_CBZ/I_AgNCs and the concentration of CBZ ranging from 0.3 nM to 10 μM and a low limit of detection of 0.1 nM. Moreover, the proposed ratiometric electrochemical sensing platform also demonstrates high selectivity, good reproducibility, secular stability, and satisfactory applicability in vegetable samples.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Enzyme-induced Cu2+/Cu+ conversion as the electrochemical signal for sensitive detection of ethyl carbamate

Traditional enzyme-linked immunosorbent assay (t-ELISA) method suffers from its relatively low sensitivity or accuracy in the detection of trace level of analyte in complicated samples. In this work, to extend the application of ELISA in practical samples, a newly electrochemical immunoassay (ECIA) was developed based on an enzyme-induced Cu²⁺/Cu⁺ conversion for the determination of ethyl carbamate (EC). Wherein, three rounds of signal transformation-the catalysis of ALP enzyme, the conversion of Cu²⁺/Cu⁺ and signal output of square wave voltammetry (SWV), can be realized to obtain higher sensitivity as compared to t-ELISA. The ECIA method combines the advantages of electrochemistry and ELISA, behaving superior detection performance, such as good selectivity, high sensitivity, and low background signal. For the wine samples, the method showed a linear detection range from 2.5 nM to 2.5 × 10⁴ nM with a limit of detection of 2.28 nM (S/N = 3), which reveals that the ECIA sensor is a promising platform for the detection of trace level of EC in practical samples.