ZIF-8 encapsulated-enzymes integrated nanozyme cascade biocatalysis platform for the colorimetric sensing of glucose and lactose in milk.

Cascade biocatalytic reactions have a wide range of applications, especially in the filed of food analysis. Herein, a multi-enzyme composite (ZGGPC) was prepared by in-situ synthesis of Zeolite imidazole framework-8 (ZIF-8) on Prussian blue (PB) modified carbon cloth (CC). The composite encapsulated both glucose oxidase and ß-galactosidase simultaneously during the synthesis process. CC and ZIF-8 showed high loading capacity for PB and natural enzymes, respectively. And ZIF-8 also displayed excellent tolerance in protecting enzyme activity under extreme conditions. Based on the cascade biocatalysis, ZGGPC was used to detect glucose and lactose by colorimetric method with detection limits of 1.2 µM and 1.7 mM, respectively. Benefiting from the merits of low cost, easy preparation, and good stability, the sensing system was used to successfully determine glucose and lactose in different milk samples. The present cascade biocatalysis system is hopeful to develop simple and efficient sensing platforms for food analysis.

LuMA-Functionalized Thermosensitive Hydrogel: A Versatile and Robust Dopamine-Triggered Platform for Diverse Biomolecules Sensing.

It is of great significance for the analysis of multiple biomarkers because a single biomarker is difficult to accurately achieve early diagnosis, disease course monitoring, and prognosis evaluation. Herein, a luminescence thermosensitive hydrogel was synthesized by radical polymerization using a methacrylic acid derivative monomer of luminol (LuMA) as luminescent, N-isopropylacrylamide (NIPAM) as thermosensitive monomer, and acrydite-oligonucleotides [dopamine (DA) aptamer, DNA C1, and DNA C2] as recognition elements. The combined DA based on the affinity interaction between the DA and the aptamer on the hydrogel polymer chain was electrochemically oxidized to dopamine quinone during the electrochemiluminescence (ECL) scanning, which effectively quenched the ECL signal of LuMA due to the resonance energy transfer (RET). In addition, the thermosensitive hydrogel showed swelling-collapse characteristics when the temperature was below and above the volume phase transition temperature. Undergoing the collapse process initiated by the temperature, the RET efficiency was further enhanced due to the shortened distance between the energy donor and acceptor, showing a 1.4 times signal amplification and achieving sensitive detection of DA with a limit of detection (LOD) of 1.7 × 10-10 M. For a proof of concept application, coupled with the target-induced release of DA from the DNA-magnetic beads bioconjugations based on duplex-specific nuclease (DSN)-assisted target recycling amplification strategy and DNAzyme cleavage reaction, this ECL-RET approach was successfully used to evaluate multiple targets including miRNA-141 and MUC1 with the LOD of 2.5 aM and 1.6 fg/mL, respectively. The excellent performances of the versatile and robust ECL-RET hydrogel in multiple target sensing showed potential applications in clinical diagnosis and disease therapeutic assay.

Diazo-reaction based dual-mode colorimetric-electrochemical sensing of nitrite in pickled food.

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.

Integrating resonance energy transfer with enzyme-triggered hydrolysis for ultrasensitive electrochemiluminescence detection of exosomes.

We developed an electrochemiluminescence (ECL) sensor by the combination of resonance energy transfer (RET) and enzyme-triggered hydrolysis reaction. Benefitting from the highly efficient RET in one nanostructure of the ECL luminophore, signal amplification by the DNA competitive reaction, and the quick response of alkaline phosphatase (ALP)-triggered hydrolysis reaction, the sensor exhibited a high sensitivity toward A549 cell-derived exosomes with a detection limit down to 1.22 × 103 particles mL-1. It performed well on biosamples from lung cancer patients and healthy individuals, revealing a potential application in cancer diagnosis.

A nanozyme-catalysis-based ratiometric electrochemical sensor for general detection of Cd2.

A new kind of internal reference probe was developed to construct a ratiometric electrochemical sensor for accurate and sensitive Cd2+ 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 H2O2. It was found that DAP was further reduced on the electrode with a constant current intensity whether Cd2+ was added or not, which enabled it to function as an excellent internal reference probe. Thus, the ratio between the reduction currents of Cd2+ and DAP (iCd2+/iDAP) was proposed to monitor Cd2+ 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 Cd2+, facilitating the quantification of Cd2+ concentration in different water samples with good recoveries. The novel internal reference probe proposed here shows great possibilities for accurate and sensitive Cd2+ detection based on the ratiometric strategy.

Mimetic peroxidase based on a gold amalgam for the colorimetric sensing of trace mercury(II) in water samples.

The development of an efficient protocol to achieve the precise monitoring of trace Hg2+ is of great significance. In this work, a colorimetric sensing platform for Hg2+ 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 Hg2+ over a wide range of 3.2 × 10-10 M-1.0 × 10-6 M. AuNPs/H-rGO/CC exhibited high selectivity towards Hg2+ due to the high special affinity between Au NPs and Hg2+. The sensing platform was further successfully applied to the determination of Hg2+ 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 Hg2+ for a variety of relevant applications.

The combination of highly efficient resonance energy transfer in one nanocomposite and ferrocene-quenching for ultrasensitive electrochemiluminescence bioanalysis.

Sensitive and accurate detection of prostate-specific antigen (PSA) is of great significance since it is regarded as a biomarker for prostate diseases. Herein, a facile strategy for the design of highly efficient electrochemiluminescence (ECL) sensor was proposed for PSA assay. Carboxylated graphitic carbon nitride (g-C3N4) nanosheet (CCN) and tris (2, 2'-Bipyridyl) ruthenium (II) (Ru(bpy)32+) encapsulated in silica nanospheres (RuSi NPs) were employed as the donor and acceptor, respectively. CCN and RuSi NPs were covalently bound within one nanocomposite (CCN@RuSi) through the amide bond, which greatly shortened the electron-transfer path. Thus, the resonance energy transfer (RET) efficiency was remarkably increased, providing a high initial ECL intensity for the ECL assay. After the successive introducing of aptamer, PSA, and ferroceneboronic acid (FcBA) on the surface of CCN@RuSi modified electrode, the ECL signal remarkably decreased, which was caused by the steric hindrance of PSA and electron transfer quenching between Fc+ and excited-state Ru(bpy)32+*. Therefore, a highly efficient ECL platform was constructed, which achieved the ultrasensitive detection of PSA with a linear range and a limit of detection of 100 fg/mL - 50 ng/mL and 1.2 fg/mL, respectively. Furthermore, the dual-affinity of the aptamer and FcBA to PSA endowed the sensor with a high selectivity for the determination of PSA in human serum samples. The present work provides an important reference for the integration of RET and quenching strategy in the ECL study with rapid, ultrasensitive, and highly selective detection performances.

Three-dimensional ordered macroporous imprinted polymer for bisphenol A recognition.

A novel kind of three-dimensional ordered macroporous molecular imprinted polymer (3DOM MIP) was prepared and studied. Monodisperse silica microspheres were used to form silica crystal template via simple centrifuge. In the presence of template molecule, acrylamide and trimethylolpropane trimethacrylate were co-polymerized in the interstices of crystal template bisphenol A (BPA). Hydrofluoric acid were employed to etch silica crystal and the mixed solvent of methanol with acetic acid were employed to extract template molecule. The results of SEM and FTIR confirmed the successful synthesis of 3DOM MIP. The obtained 3DOM MIP exhibited a rapid adsorption kinetics and a specific adsorption capacities toward template molecule because of the small size of MIP wall, which possessed much more effective imprinted cavies. Meanwhile, 3DOM MIP could selective recognized BPA from its structural analogues.

Specifically triggered dissociation based ratiometric electrochemical sensor for H2O2 measurement in food samples.

A novel ratiometric strategy based electrochemical sensor was developed to quantitative assay of H2O2 in different food samples. 4-aminophenylboronic acid pinacol ester (ABAPE) dissociation was specifically triggered by H2O2 to generate electro-active 4-aminophenol (4-AP), which not only can be oxidized to indirectly indicate the concentration of H2O2, but also endowed the sensor with high selectivity. Meanwhile, a reference probe of poly(thionine) (TH) was modified with ketjen black (KB) and gold nanoparticles (AuNPs) on electrode surface. KB and AuNPs displayed high electrocatalytic activity to 4-AP. A current ratio between 4-AP and TH (i/iTH) showed a good linear relationship with the concentration of H2O2 in a range of 3.0 × 10-7 - 1.0 × 10-4 mol/L (0.010 ppm - 3.40 ppm) with a limit of detection of 2.6 × 10-7 mol/L (0.009 ppm) (S/N = 3). Moreover, the ratiometric strategy based sensor possessed good accuracy, reliability, and stability, and successfully determined H2O2 in food samples with satisfactory results.

Sensitive and selective "signal-off" electrochemiluminescence sensing of prostate-specific antigen based on an aptamer and molecularly imprinted polymer.

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.

Dual-recognition colorimetric sensing of thrombin based on surface-imprinted aptamer-Fe3O4.

Thrombin plays an essential role in blood coagulation and some physiological and pathological processes. The convenient, rapid, sensitive, and specific detection of thrombin is of great significance in clinical research and diagnosis. Herein, surface molecularly imprinted polymer (MIP) was modified on aptamer-functionalized Fe3O4 nanoparticles (MIP-aptamer-Fe3O4 NP) for thrombin colorimetric assay by taking advantage of the peroxidase-like activity of Fe3O4 NP. With the adsorption of thrombin into imprinted cavities, the exposed surface area of Fe3O4 NP decreased, causing a decrease in its peroxidase-like activity toward 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. On the other hand, the reductive amino acids on the thrombin surface also impeded the oxidation of TMB. Both phenomena caused the light blue color of the sensing solution. Thus, a specifically sensitive colorimetric approach for the visual detection of thrombin was proposed with a linear range and limit of detection of 108.1 pmol L-1-2.7 × 10-5 mol L-1 and 27.8 pmol L-1, respectively. Moreover, due to the double recognition elements of MIP and aptamer, the prepared MIP-aptamer-Fe3O4 NP showed higher selectivity to thrombin than that based on only one recognition element. It is worth noting that no special property (e.g. electrochemical or fluorescence activity) of the template was required in this work. Thus, more template molecules can be easily, selectively, and sensitively detected based on the proposed MIP-aptamer-mimic enzyme colorimetric sensing strategy.

Carboxylation modified meso-porous carbon aerogel templated by ionic liquid for solid-phase microextraction of trace tetracyclines residues using HPLC with UV detection.

A carbon aerogel composite templated and catalyzed by ionic liquid was fabricated to obtain a meso-porous and cross-linked structure while avoiding the freeze and supercritical drying. It was then carboxylated to obtain favorable surface groups. The easily prepared material displayed excellent extraction effect of six tetracyclines (TCs) compared to the non-carboxylated carbon aerogel. A direct immersion solid-phase microextraction method to determine six TCs in aqueous samples was developed coupling with high-performance liquid chromatography (HPLC) with UV-Vis detector set at 355 nm. The experimental parameters affecting the analytical performance of this method, including sample pH, ionic strength, extraction and desorption time, extraction volume, and temperature, were optimized. Adsorption kinetics and thermodynamics models were used to clarify the extraction mechanism. Under the optimized conditions, this method has a wide linear range of 2-1000 µg L-1, low limits of detection of 0.36-0.71 µg L-1, repeatability of 1.85-10.96%, and reproducibility of 4.92-13.47% for six TCs. The method was successfully applied to detect TC residues in egg and poultry farm wastewater samples.

Carbon aerogel as a solid-phase microextraction fiber coating for the extraction and detection of trace tetracycline residues in food by coupling with high-performance liquid chromatography.

A direct immersion solid-phase microextraction method for determining tetracyclines (TCs) was developed by coupling with high-performance liquid chromatography. A carbon aerogel (CA) was synthesized as a fiber coating with high extractive properties and a low density of 0.1855 g cm-3via ambient pressure drying and carbonization. The as-synthesized CA exhibited a high specific surface area and a cross-linked structure; it was characterized via scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and Brunauer-Emmett-Teller analysis, etc. The extraction performance for six TCs was investigated, and the main experimental parameters were optimized by the Box-Behnken design. Adsorption kinetics, Langmuir and Freundlich models were used to clarify the extraction mechanism. This method showed wide linear ranges of 1-500 µg L-1, low limits of detection of 0.52-1.05 µg L-1, good repeatability of 1.37-12.47%, and satisfactory inter-fiber reproducibility of 8.51-15.81% relative standard deviation for the detection of six TCs. Moreover, this study provided an interesting insight into the detection of TCs residues in food samples.

Sensitive and selective detection of glycoprotein based on dual-signal and dual-recognition electrochemical sensing platform.

Glycoproteins play extraordinary roles in biology and clinic. The specifically sensitive detection of glycoproteins by electrochemical methods is still a challenging task due to their poor electro-activity and sensitive nature to environment. In this work, ovalbumin (OVA), a model glycoprotein, was sensitively detected by a molecularly imprinted polymer (MIP) based electrochemical sensor, which was prepared by electropolymerizing 3-thiophene boric acid in the presence of OVA. Due to boronate affinity, the rebound OVA interacted with ferrocene boric acid (Fc-BA) to construct a sandwich structural sensing platform. Dual-recognition elements, imprinted effect and the boronate affinity, enabled the sensor to recognize OVA from other proteins. The rebinding of OVA caused the current changes of thionine and Fc-BA, which were combined as a dual-signal for OVA sensitive detection with a low limit of detection of 0.82 pg/mL (S/N = 3). The good performances of sensor indicated its potential applications in clinical diagnosis and other related fields.

A boronic acid carbon nanodots/poly(thionine) sensing platform for the accurate and reliable detection of NADH.

In this work, a novel electrochemical sensing platform was designed and fabricated by the modification of boronic acid functionalized carbon nanodots (B-CNDs) and poly(thionine) (pTHI) on an electrode surface. B-CNDs can not only accelerate electron transfer but also covalently interact with cis-diol groups of dihydronicotinamide adenine dinucleotide (NADH) through functionalized boronic acid groups. Meanwhile, pTHI served as an inner reference element to provide a built-in correction, which enabled the sensor to detect NADH with high accuracy and reliability based on a ratiometric signal (∆INADH/∆ITHI). The electrochemical experimental results demonstrated that the ratiometric strategy-based sensor possessed good selectivity and high sensitivity. A linear range of 5.0 × 10-7 - 2.0 × 10-4 mol/L for NADH detection was obtained with a limit of detection of 1.5 × 10-7 mol/L. The sensor has been applied to analyze NADH in human serum samples with satisfactory results. The simple and effective ratiometric strategy reported here can be further used to prepare electrochemical sensors for selective, sensitive, and reliable detection of other cis-diol compounds.

Au-polythionine nanocomposites: a novel mediator for bisphenol A dual-signal assay based on imprinted electrochemical sensor.

In this work, a novel electrochemical sensor was developed by the modification of poly(p-aminobenzene sulfonic acid) (pABSA), Au-polythionine (Au-pTH) nanowires, and molecularly imprinted polymer (MIP) on glassy carbon electrode surface for bisphenol A (BPA) detection. The results of characterizations including scanning electron microscope, transmission electron microscopy, Fourier transform infrared spectra, and X-ray diffraction showed the successful synthesis of helical structural Au-pTH nanowires, which acted as an electro-active probe for BPA detection. Cyclic voltammetry results illustrated that the modified pABSA, Au-pTH, and MIP endowed the sensor with good electrocatalytic activity, the second current signal, and recognition ability, respectively. Since the imprinted cavities provided electron transfer channels for thionine (TH) redox, the peak current of TH can be found in a blank electrolyte. The added BPA molecules can be rebound in imprinted cavities, which are oxidized and then display its current. The rebound BPA molecules in turn blocked the electron transfer channels for TH redox, resulting in the decrease of TH current. A double signal defined, as the sum of the changes of TH current (∆iTH|) and BPA current (|∆iBPA|) (|∆iTH|+|∆iBPA|), was employed as the detected signal for BPA sensitive detection, which was linearly proportional to the logarithm of concentration of BPA ranging from 8.0 × 10-8 to 1.0 × 10-4 mol/L with a limit of detection of 3.8 × 10-8 mol/L (S/N = 3) in a weakly acidic solution. Moreover, the natural recognition ability of MIP enabled the sensor to selectively detect BPA from its analogues. The proposed dual-signal strategy-based sensor provided a feasible tool for rapid, sensitive, and selective determination of BPA. Graphical abstract.

Multilayer sensing platform: gold nanoparticles/prussian blue decorated graphite paper for NADH and H2O2 detection.

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.

A ratiometric strategy -based electrochemical sensing interface for the sensitive and reliable detection of imidacloprid.

In this study, a ratiometric strategy-based electrochemical sensor was developed by electropolymerization of thionine (THI) and ß-cyclodextrin (ß-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 ß-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.

From non-electroactive to electroactive species: highly selective and sensitive detection based on a dual-template molecularly imprinted polymer electrochemical sensor.

A new strategy for sensitive detection using a dual-template molecularly imprinted based electrochemical sensor was developed using thionine as an electrochemical probe. A single-signal for the non-electroactive template and a dual-signal for the electroactive template in the selective and sensitive detection have been achieved on the prepared sensor.

"Sign-on/off" sensing interface design and fabrication for propyl gallate recognition and sensitive detection.

A new strategy based on sign-on and sign-off was proposed for propyl gallate (PG) determination by an electrochemical sensor. The successively modified poly(thionine) (PTH) and molecular imprinted polymer (MIP) showed an obvious electrocatalysis and a good recognition toward PG, respectively. Furthermore, the rebound PG molecules in imprinted cavities not only were oxidized but also blocked the electron transmission channels for PTH redox. Thus, a sign-on from PG current and a sign-off from PTH current were combined as a dual-sign for PG detection. Meanwhile, the modified MIP endowed the sensor with recognition capacity. The electrochemical experimental results demonstrated that the prepared sensor possessed good selectivity and high sensitivity. A linear ranging from 5.0×10(-8) to 1.0×10(-4)mol/L for PG detection was obtained with a limit of detection of 2.4×10(-8)mol/L. And the sensor has been applied to analyze PG in real samples with satisfactory results. The simple, low cost, and effective strategy reported here can be further used to prepare electrochemical sensors for other compounds selective recognition and sensitive detection.