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Development of an effective and convenient sensor for sensitive detection of nitrites is of great concern since excessive amounts of nitrites can be harmful to both human health and the environment. In this work, Cu-MOF modified exfoliated graphite paper (EGP) was employed as a signal reporter to enable the visual and electrochemical dual-mode sensing of nitrites. Cu-MOFs were in situ synthesized on EGP, which exhibited an excellent oxidase enzyme-like activity to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidation product (oxTMB). The multi-layer structure and the superior electrical conductivity of EGP not only facilitated the loading of the Cu-MOF nanozyme for colorimetric sensing but also enabled its use as an underlying backbone to support electroanalysis. Based on the recognition of nitrite through a highly specific diazo reaction between nitrite and oxTMB, the addition of nitrite caused the colorimetric sensing solution to change color from blue to green, which allowed for the colorimetric sensing of nitrite with a limit of detection (LOD) of 8.5 × 10 -6 mol L -1 . Meanwhile, the Cu-MOF/EGP electrochemical platform was employed for ratiometric detection of nitrite based on the electrochemical oxidation of nitrite and TMB. Compared with the colorimetric mode, the electrochemical mode possessed higher sensitivity with a LOD of 5.4 × 10 -7 mol L -1 , indicating the high sensitivity and accuracy of the proposed dual-mode sensing strategy. Furthermore, the determination of nitrite in different pickled food samples is demonstrated.

A new kind of internal reference probe was developed to construct a ratiometric electrochemical sensor for accurate and sensitive Cd 2+ detection. AuPt nanoclusters (AuPt NCs) were synthesized and then evenly distributed on reduced graphene oxide (rGO) to form a nanocomposite (AuPt-rGO), which showed peroxidase-like activity and catalyzed the oxidation of o -phenylenediamine (OPD) to 2,3-diaminophenazine (DAP) in the presence of H 2 O 2 . It was found that DAP was further reduced on the electrode with a constant current intensity whether Cd 2+ was added or not, which enabled it to function as an excellent internal reference probe. Thus, the ratio between the reduction currents of Cd 2+ and DAP ( i Cd 2+ / i DAP ) was proposed to monitor Cd 2+ based on the ratiometric strategy. Additionally, the high surface area and good conductivity of AuPt-rGO endowed the sensor with enhanced sensitivity. A wide linear range and a low limit of detection were measured at 5.0 × 10 -8 -1.0 × 10 -4 mol L -1 and 3.1 × 10 -8 mol L -1 , respectively. Compared with the single signal sensors, this ratiometric sensor presents more accuracy and reliability in the detection of Cd 2+ , facilitating the quantification of Cd 2+ concentration in different water samples with good recoveries. The novel internal reference probe proposed here shows great possibilities for accurate and sensitive Cd 2+ detection based on the ratiometric strategy.

The development of an efficient protocol to achieve the precise monitoring of trace Hg 2+ is of great significance. In this work, a colorimetric sensing platform for Hg 2+ monitoring was constructed via modifying gold nanoparticles (Au NPs) and hemin/reduced graphene oxide (H-rGO) onto carbon cloth (CC) (AuNPs/H-rGO/CC). The Au NPs, anchored on the surface of the interlaced fibers of CC without aggregation, showed high peroxidase-mimetic catalytic activity. The formed Au amalgam and the synergistic effect of Au NPs and H-rGO enhanced the peroxidase-mimetic activity of AuNPs/H-rGO/CC and enabled the sensing platform to achieve the colorimetric detection of Hg 2+ over a wide range of 3.2 × 10 -10 M-1.0 × 10 -6 M. AuNPs/H-rGO/CC exhibited high selectivity towards Hg 2+ due to the high special affinity between Au NPs and Hg 2+ . The sensing platform was further successfully applied to the determination of Hg 2+ in different water samples via a simplified operation involving just tweezers. The present sensing protocol is expected to provide an efficient way to construct colorimetric sensors and to promote the sensitive determination of Hg 2+ for a variety of relevant applications.

Specific and sensitive determination of prostate-specific antigen (PSA) in complex real samples holds significant importance as it is an effective molecular biomarker for the clinical diagnosis of prostate cancer. Herein, we constructed a dual-recognition electrochemiluminescence (ECL) sensor based on both the recognition elements of an aptamer and molecularly imprinted polymers (MIP) for the selective and ultrasensitive determination of PSA. The aptamer was self-assembled on gold nanoparticle (AuNP) modified electrodes through Au-S bonds. Subsequently, a layer of MIP membrane was synthesized by electropolymerization of dopamine (DA) to fabricate an aptamer-MIP sensor. After the rebinding of PSA onto imprinted cavities, the ECL response of luminol in the solution decreased. This "signal-off" strategy was employed for PSA detection with a wide linear range and a low limit of detection of 5 pg mL -1 -50 ng mL -1 and 3.0 pg mL -1 , respectively. Compared with individual aptamer sensors, the dual-recognition sensor showed higher specific recognition ability for the determination of PSA. Meanwhile, the good stability, reproducibility, and regenerability endowed the dual recognition sensor with favorable application value in early clinical diagnosis.

In this study, an exfoliated graphite paper (EGP) was employed as a supported electrode for prussian blue (PB) and gold nanoparticles (AuNPs) modification to fabricate a novel electrochemical sensor. The scanning electron microscopy image showed the multilayer structure of the sensor with PB and AuNPs uniform coating. Due to the high surface area and good conductivity of the multilayer structure, as well as the conductivity of AuNPs, the prepared sensor exhibited an obvious improvement for sensitive detection of nicotinamide adenine dinucleotide (NADH) with a linear range of 5.0 × 10-7 mol L-1-1.0 × 10-3 mol L-1. Moreover, the modified PB film endowed the sensor with good electrocatalytic activity for hydrogen peroxide (H2O2) sensing. Under the optimal conditions, the linear range and limit of detection for H2O2 sensitive detection were 5.0 × 10-8 mol L-1-1.0 × 10-3 mol L-1 and 4.8 × 10-9 mol L-1, respectively. The sensor can also be used for detection in real samples with satisfactory results. The advantages of low-cost, easy modification, multilayer structure, and good conductivity made the EGP an alternative candidate for supported electrode materials in the preparation and application of various electrochemical sensors.

In this study, a ratiometric strategy-based electrochemical sensor was developed by electropolymerization of thionine (THI) and β-cyclodextrin (β-CD) composite films on a glassy carbon electrode surface for imidacloprid (IMI) detection. THI played the role of an inner reference element to provide a built-in correction. In addition, the modified β-CD showed good selective enrichment for IMI to improve the sensitivity and anti-interference ability of the sensor. The current ratio between IMI and THI was calculated as the detected signal for IMI sensing. Compared with common single-signal sensing, the proposed ratiometric strategy showed a higher linear range and a lower limit of detection of 4.0 × 10-8-1.0 × 10-5 mol L-1 and 1.7 × 10-8 mol L-1, respectively, for IMI detection. On the other hand, the ratiometric strategy endowed the sensor with good accuracy, reproducibility, and stability. The sensor was also used for IMI determination in real samples with satisfactory results. The simple, effective, and reliable way reported in this study can be further used to prepare ratiometric strategy-based electrochemical sensors for the selective and sensitive detection of other compounds with good accuracy and stability.

Multi-walled carbon nanotubes (MWNTs) and boronic acid based molecular imprinting polymer (MIP) were successively modified on a glassy carbon electrode surface to fabricate a novel electrochemical sensor for rutin recognition and detection. 3-Aminophenylboronic acid (APBA) was chosen as a monomer for the electropolymerization of MIP film in the presence of rutin. In addition to the imprinted cavities in MIP film to complement the template molecule in shape and functional groups, the high affinity between the boronic acid group of APBA and vicinal diols of rutin also enhanced the selectivity of the sensor, which made the sensor display a good selectivity to rutin. Moreover, the modified MWNTs improved the sensitivity of the sensor for rutin detection. The mole ratios of rutin and APBA, electropolymerized scan cycles and rates, and pH value of the detection solution were optimized. Under optimal conditions, the sensor was used to detect rutin in a linear range from 4.0 × 10 -7 to 1.0 × 10 -5 mol L -1 with a detection limit of 1.1 × 10 -7 mol L -1 . The sensor has also been applied to assay rutin in tablets with satisfactory results.

A facile electrochemical sensor for hydroquinone (HQ) and catechol (CC) determination was successfully fabricated by the modification of poly(3-aminophenylboronic acid) (pAPBA) film and multi-walled carbon nanotubes (MWCNTs) on a glassy carbon electrode (pAPBA/MWCNTs/GCE). The prepared sensor was characterized by scanning electron microscope and electrochemical impedance spectroscopy. Under optimal conditions, differential pulse voltammetry was employed to quantify individual HQ and CC within the concentration range of 5.0 × 10(-7)-4.0 × 10(-5) mol L(-1) and 7.0 × 10(-6)-1.0 × 10(-4) mol L(-1), respectively. Based on the covalent binding between the boronic acid groups of pAPBA film and the cis-diol-containing molecule, a novel substitution-sensing strategy was proposed for the highly sensitive determination of CC. With the addition of CC into HQ solution, covalent interaction between CC and APBA occurred and the HQ was displaced by CC, resulting in a decrease of HQ oxidation peak current and the increase of the CC oxidation peak current. The summation of both current changes (Δ|IHQ| + Δ|ICC|) were combined for CC sensitive detection in a concentration range of 4.0 × 10(-8)-1.7 × 10(-5) mol L(-1) with a limit of detection of 4.3 × 10(-9) mol L(-1). The sensor was successfully applied to the determination of CC in spiked water samples.

A novel electrochemical sensor for bovine hemoglobin (BHb) recognition and detection was prepared by a surface molecularly imprinted technique. Aldehyde group-functionalized silica microspheres were modified on an Au electrode surface, and these were subsequently covalently bound with the template molecule, BHb, through imine bonds. Electropolymerization was performed to deposit polypyrrole onto the above modified electrode surface. A three-dimensional macroporous structural sensor was obtained after etching of silica and the extraction of BHb. Differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the fabrication process of the sensor using [Fe(CN)6](3-)/[Fe(CN)6](4-) as an electroactive probe. Since all imprinted cavities were situated at the surface of the polymers, the prepared sensor exhibited considerably fast binding kinetics. Compared to other non-template proteins, the sensor showed an excellent recognition capacity for BHb. Moreover, the prepared sensor also exhibited a dependent relationship between the concentration of BHb and the peak current of [Fe(CN)6](3-)/[Fe(CN)6](4-).