Colorimetric array sensor based on bimetallic nitrogen-doped carbon-based nanozyme material to detect multiple antioxidants.

Copper-cobalt bimetallic nitrogen-doped carbon-based nanoenzymatic materials (CuCo@NC) were synthesized using a one-step pyrolysis process. A three-channel colorimetric sensor array was constructed for the detection of seven antioxidants, including cysteine (Cys), uric acid (UA), tea polyphenols (TP), lysine (Lys), ascorbic acid (AA), glutathione (GSH), and dopamine (DA). CuCo@NC with peroxidase activity was used to catalyze the oxidation of TMB by H2O2 at three different ratios of metal sites. The ability of various antioxidants to reduce the oxidation products of TMB (ox TMB) varied, leading to distinct absorbance changes. Linear discriminant analysis (LDA) results showed that the sensor array was capable of detecting seven antioxidants in buffer and serum samples. It could successfully discriminate antioxidants with a minimum concentration of 10 nM. Thus, multifunctional sensor arrays based on CuCo@NC bimetallic nanoenzymes not only offer a promising strategy for identifying various antioxidants but also expand their applications in medical diagnostics and environmental analysis of food.

Multifunctional N, Fe-doped carbon dots with peroxidase-like activity for the determination of H2O2 and ascorbic acid and cell protection against oxidation.

Multifunctional N, Fe-doped carbon dots (N, Fe-CDs) were synthesized by the one-step hydrothermal method using ferric ammonium citrate and dicyandiamide as raw materials. The N, Fe-CDs exhibited peroxidase-like (POD) activity by catalyzing the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) to the green oxidation state ox-TMB in the presence of hydrogen peroxide (H2O2). Subsequently, based on the POD activity of N, Fe-CDs, an efficient and sensitive colorimetric method for the detection of H2O2 and ascorbic acid (AA) was established with a limit of detection of 0.40 µM and 2.05 µM. The proposed detection method has been successfully applied to detect AA in fruit juice, vitamin C tablets, and human serum samples and has exhibited excellent application prospects in biotechnology and food fields. Furthermore, N, Fe-CDs also showed a protective effect on the cell damage caused by H2O2 and could be used as an antioxidant agent.

Superior oxidase-mimetic activity of FeCo-NC dual-atom nanozyme for smartphone-based visually colorimetric assay of organophosphorus pesticides.

A colorimetric analysis platform has been successfully developed based on FeCo-NC dual-atom nanozyme (FeCo-NC DAzyme) for the detection of organophosphorus pesticides (OPPs). The FeCo-NC DAzyme exhibited exceptional oxidase-like activity (OXD), enabling the catalysis of colorless TMB to form blue oxidized TMB (oxTMB) without the need for H2O2 involvement. By combining acid phosphatase (ACP) hydrolase with FeCo-NC DAzyme, a "FeCo-NC DAzyme + TMB + ACP + SAP" colorimetric system was constructed, which facilitated the rapid detection of malathion. The chromogenic system was applied to detect malathion using a smartphone-based app and an auxiliary imaging interferogram device for colorimetric measurements, which have a linear range of 0.05-4.0 µM and a limit of detection (LOD) as low as 15 nM in real samples, comparable to UV-Vis and HPLC-DAD detection methods. Overall, these findings present a novel approach for convenient, rapid, and on-site monitoring of OPPs.

Popcorn-like bimetallic palladium/platinum exhibiting enhanced peroxidase-like activity for signal enhancement in lateral flow immunoassay.

BACKGROUND: The lateral flow immunoassay (LFIA) is widely employed as a point-of-care testing (POCT) technique. However, its limited sensitivity hinders its application in detecting biomarkers with low abundance. Recently, the utilization of nanozymes has been implemented to enhance the sensitivity of LFIA by catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The catalytic performance of nanozymes plays a crucial role in influencing the sensitivity of LFIA. RESULTS: The Cornus officinalis Sieb. et Zucc-Pd@Pt (CO-Pd@Pt) nanozyme with good peroxidase-like activity was synthesized herein through a facile one-pot method employing Cornus officinalis Sieb. et Zucc extract as a reducing agent. The morphology and composition of the CO-Pd@Pt nanozyme were characterized using TEM, SEM, XRD, and XPS. As a proof of concept, the as-synthesized CO-Pd@Pt nanozyme was utilized in LFIA (CO-Pd@Pt-LFIA) for the detection of human chorionic gonadotropin (hCG). Compared to conventional gold nanoparticles-based LFIA (AuNPs-LFIA), CO-Pd@Pt-LFIA demonstrated a significant enhancement in the limit of detection (LOD, 0.08 mIU/mL), which is approximately 160 times lower than that of AuNPs-LFIA. Furthermore, experiments evaluating accuracy, precision, selectivity, interference, and stability have confirmed the practical applicability of CO-Pd@Pt-LFIA for hCG content determination. SIGNIFICANCE: The present study presents a novel approach for the synthesis of bimetallic nanozymes through environmentally friendly methods, utilizing plant extracts as both protective and reducing agents. Additionally, an easily implementable technique is proposed to enhance signal detection in lateral flow immunoassays.

Hexagonal Boron Nitride Quantum Dots Embedded on Layer-by-Layer Films for Peroxidase-Assisted Colorimetric Detection of ß-Galactosidase Producing Pathogens.

Pathogen detection has become a major research area all over the world for water quality surveillance and microbial risk assessment. Therefore, designing simple and sensitive detection kits plays a key role in envisaging and evaluating the risk of disease outbreaks and providing quality healthcare settings. Herein, we have designed a facile and low-cost colorimetric sensing strategy for the selective and sensitive determination of ß-galactosidase producing pathogens. The hexagonal boron nitride quantum dots (h-BN QDs) were established as a nanozyme that showed prominent peroxidase-like activity, which catalyzes 3,3',5,5'-tetramethylbenzidine (TMB) oxidation by H2O2. The h-BN QDs were embedded on a layer-by-layer assembled agarose biopolymer. The ß-galactosidase enzyme partially degrades ß-1,4 glycosidic bonds of agarose polymer, resulting in accessibility of h-BN QDs on the solid surface. This assay can be conveniently conducted and analyzed by monitoring the blue color formation due to TMB oxidation within 30 min. The nanocomposite was stable for more than 90 days and was showing TMB oxidation after incubating it with Escherichia coli (E. coli). The limit of detection was calculated to be 1.8 × 106 and 1.5 × 106 CFU/mL for E. coli and Klebsiella pneumonia (K. pneumonia), respectively. Furthermore, this novel sensing approach is an attractive platform that was successfully applied to detect E. coli in spiked water samples and other food products with good accuracy, indicating its practical applicability for the detection of pathogens in real samples.

Cu-BTC Derived Mesoporous CuS Nanomaterial as Nanozyme for Colorimetric Detection of Glutathione.

In this paper, Cu-BTC derived mesoporous CuS nanomaterial (m-CuS) was synthesized via a two-step process involving carbonization and sulfidation of Cu-BTC for colorimetric glutathione detection. The Cu-BTC was constructed by 1,3,5-benzenetri-carboxylic acid (H3BTC) and Cu2+ ions. The obtained m-CuS showed a large specific surface area (55.751 m2/g), pore volume (0.153 cm3/g), and pore diameter (15.380 nm). In addition, the synthesized m-CuS exhibited high peroxidase-like activity and could catalyze oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine to a blue product. Peroxidase-like activity mechanism studies using terephthalic acid as a fluorescent probe proved that m-CuS assists H2O2 decomposition to reactive oxygen species, which are responsible for TMB oxidation. However, the catalytic activity of m-CuS for the oxidation of TMB by H2O2 could be potently inhibited in the presence of glutathione. Based on this phenomenon, the colorimetric detection of glutathione was demonstrated with good selectivity and high sensitivity. The linear range was 1-20 µM and 20-300 µM with a detection limit of 0.1 µM. The m-CuS showing good stability and robust peroxidase catalytic activity was applied for the detection of glutathione in human urine samples.

A label-free colorimetric biosensor utilizing natural material for highly sensitive exosome detection.

Exosomes, extracellular vesicles secreted by cells, play a crucial role in intercellular communication by transferring information from source cells to recipient cells. These vesicles carry important biomarkers, including nucleic acids and proteins, which provide valuable insights into the parent cells' status. As a result, exosomes have emerged as noninvasive indicators for the early diagnosis of cancer. Colorimetric biosensors have garnered significant attention due to their cost-effectiveness, simplicity, rapid response, and reproducibility. In this study, we employ sporopollenin microcapsules (SP), a natural biopolymer material derived from pollen, as a substrate for gold nanoparticles (AuNPs). By modifying the SP-Au complex with CD63 aptamers, we develop a label-free colorimetric biosensor for exosome detection. In the absence of exosomes, the SP-Au complex catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), resulting in a color change from colorless to blue. However, the addition of exosomes inhibits the catalytic activity of the SP-Au complex due to coverage of exosomes on AuNPs. This colorimetric biosensor exhibits high sensitivity and selectivity for exosome detection, with a detection limit of 10 particles/µL and a wide linear range of 10 - 108 particles/µL. Additionally, the SP-Au biosensor demonstrates remarkable resistance to serum protein adsorption and excellent catalytic stability even in harsh environments, making it highly suitable for clinical diagnostics.

An Au bipyramids@CuZn MOF core-shell nanozyme enables universal SERS and a colorimetric dual-model bioassay.

In this study, a new type of gold nano-bipyramids@CuZn bimetallic organic framework (AuNBPs@CuZn MOF) nanozyme with high peroxidase (POD)-like activity and surface enhanced Raman scattering (SERS) activity was constructed with a special core-shell structure, which can catalyze the oxidation of TMB (colourless and Raman-inactive) into ox-TMB (blue and Raman-active). An AuNBPs@CuZn MOF-enabling universal SERS and colorimetric dual-model bioassay was thus developed for biomolecules with excellent performance, and has promising application prospects in the biosensing fields.

A metal-phenolic coordination framework nanozyme exhibits dual enzyme mimicking activity and its application is effective for colorimetric detection of biomolecules.

Biomolecules play vital roles in many biological processes and diseases, making their identification crucial. Herein, we present a colorimetric sensing method for detecting biomolecules like cysteine (Cys), homocysteine (Hcy), and glutathione (GSH). This approach is based on a reaction system whereby colorless 3,3',5,5'-tetramethylbenzidine (TMB) undergoes catalytic oxidation to form blue-colored oxidized TMB (ox-TMB) in the presence of hydrogen peroxide (H2O2), utilizing the peroxidase and catalase-mimicking activities of metal-phenolic coordination frameworks (MPNs) of Cu-TA, Co-TA, and Fe-TA nanospheres. The Fe-TA nanospheres demonstrated superior activity, more active sites and enhanced electron transport. Under optimal conditions, the Fe-TA nanospheres were used for the detection of biomolecules. When present, biomolecules inhibit the reaction between TMB and H2O2, causing various colorimetric responses at low detection limits of 0.382, 0.776 and 0.750 µM for Cys, Hcy and GSH. Furthermore, it was successfully applied to real water samples with good recovery results. The developed sensor not only offers a rapid, portable, and user-friendly technique for multi-target analysis of biomolecules at low concentrations but also expands the potential uses of MPNs for other targets in the environmental field.

Colorimetric aptasensor based on temporally controllable light-stimulated oxidase-mimicking fluorescein for the sensitive detection of exosomes in mild conditions.

Analysis of exosomes provides important information for rapid and non-invasive screening of tumors. However, sensitive and convenient detection of exosomes remains technically challenging to date. Herein, a colorimetric aptasensor based on the light-stimulated oxidase-mimicking activity of FITC was constructed for detecting ovarian cancer (OC) exosomes. The aptasensor contained an EpCAM aptamer to capture OC exosomes. Cholesterol and fluorescein (FITC) were used to modify either end of the DNA (DNA anchor). The DNA anchor could combine with exosomes through a hydrophobic reaction between cholesterol and the lipid membrane. FITC oxidized 3,3',5,5'-tetramethylbenzidine (TMB) under a 365 nm LED light source in a temporally controllable manner under mild conditions, causing the solution to change from colorless to blue, and the corresponding UV-vis absorbance increased. Based on this principle, the exosomes were qualitatively analyzed by observing the color change with the naked eye. In parallel, the exosome concentration was also detected using UV-vis spectrophotometry. The linear range was from 2 × 105 to 100 × 105 particles per mL with a limit of detection of 1.77 × 105 particles per mL. The developed aptasensor also exhibited favorable selectivity and could discriminate the exosomes from OC cells and normal cells. Besides, the receiver operating characteristic (ROC) curve demonstrates that it is possible to distinguish between patients with OC and healthy donors (HDs) using exosomes as the biomarker. Our technology may expand the applications of DNA-based detection method-enabled OC diagnostic tools.

MOF-derived nanozyme CuOx@C and its application for cascade colorimetric detection of phytosterols.

Phytosterols (PSs), a class of naturally occurring bioactive lipid compounds, have been found to possess a significant cholesterol-lowering effect. In developing countries, the consumption of rapeseed oil is the primary pathway of PS intake for the general population. However, developing low-cost, real-time, and high-throughput screening techniques for PSs remains a challenge. Here, a Cu-based nanocomposite CuOx@C was synthesized via a simple method of the calcination of HKUST-1 and systematically characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The CuOx@C demonstrated excellent peroxidase-like (POD-like) activity, functioning as a peroxidase mimic to facilitate the catalysis of 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidized form (oxTMB), thereby initiating a discernible color response. On the basis of this discovery, a CuOx@C-based colorimetric method for detecting total sterols in rapeseed was successfully constructed via cascade reactions. After optimizing the conditions, the high-throughput screening of total sterols in rapeseed could be completed in only 21 min, which significantly facilitated the sensing of PSs. A linear range of 0.6-6 mg/g was achieved for the detection of total sterols in rapeseed samples, thereby satisfying the requirements for detection. In addition, due to the high stability of CuOx@C and the specificity of cholesterol oxidase, the developed method had excellent stability and selectivity toward PSs, indicating that this work has huge prospects for commercial application. This innovative work overcomes the limitation of the instrumental method and provides a portable and reliable tool for total sterols detection. It can also facilitate the development of oilseeds with a high content of PSs.

Multifunctional 2D hemin-bridged MOF for the efficient removal and dual-mode detection of organophosphorus pesticides.

The high-residual and bioaccumulation property of organophosphorus pesticides (OPs) creates enormous risks towards the ecological environment and human health, promoting the research for smart adsorbents and detection methods. Herein, 2D hemin-bridged MOF nanozyme (2D-ZHM) was fabricated and applied to the efficient removal and ultrasensitive dual-mode aptasensing of OPs. On the one hand, the prepared 2D-ZHM contained Zr-OH groups with high affinity for phosphate groups, endowing it with selective recognition and high adsorption capacity for OPs (285.7 mg g-1 for glyphosate). On the other hand, the enhanced peroxidase-mimicking biocatalytic property of 2D-ZHM allowed rapid H2O2-directed transformation of 3,3',5,5'-tetramethylbenzidine to oxidic product, producing detectable colorimetric or photothermal signals. Using aptamers of specific recognition capacity, the rapid quantification of two typical OPs, glyphosate and omethoate, was realized with remarkable sensitivity and selectivity. The limit of detections (LODs) of glyphosate were 0.004 nM and 0.02 nM for colorimetric and photothermal methods, respectively, and the LODs of omethoate were 0.005 nM and 0.04 nM for colorimetric and photothermal methods, respectively. The constructed dual-mode aptasensing platform exhibited outstanding performance for monitoring OPs in water and fruit samples. This work provides a novel pathway to develop MOF-based artificial peroxidase and integrated platform for pollutant removal and multi-mode aptasensing.

Hollow versatile Ag@Pt alloy nanoparticles with nanozyme activity for detection and photothermal sterilization of Helicobacter pylori.

In view of a large number of people infected with Helicobacter pylori (H. pylori) with great harm followed, there is an urgent need to develop a non-invasive, easy-to-operate, and rapid detection method, and to identify effective sterilization strategies. In this study, highly specific nanoprobes with nanozyme activity, Ag@Pt nanoparticles (NPs) with the antibody, were utilized as a novel lateral flow immunoassay (LFIA). The optical label (Ag@Pt NPs) was enhanced by the introduction of the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) and compared with a gold nanoparticles (Au NPs) optical label. Under the optimal condition, Ag@Pt-LFIA and TMB-enhanced Ag@Pt-LFIA for H. pylori were successfully established, two of which were over twofold and 100-fold more sensitive than conventional visual Au NP-based LFIA, respectively. Furthermore, Ag@Pt NPs with the antibody irradiated with NIR laser (808 nm) at a power intensity of 550 mW/cm2 for 5 min exhibited a remarkable antibacterial effect. The nanoprobes could close to bacteria through effective interactions between antibodies and bacteria, thereby benefiting photothermal sterilization. Overall, Ag@Pt NPs provide promising applications in pathogen detection and therapeutic applications.

A convenient smartphone-assisted colorimetric for 6-Mercaptopurine detection using enhanced oxidase-like activity of ß-cyclodextrin modified MnO2 nanosheets.

6-mercaptopurine (6-MP) is widely used in the treatment of many diseases, but exhibits some serious side effects due to its toxicity. Therefore, it is important and imperative to effectively control and monitoring concentration of 6-MP. Herein, we designed a smartphone-assisted colorimetric sensing platform for 6-MP detection, based on an excellent ß-cyclodextrin modified MnO2 nanosheets (ß-CD@MnO2 NNS) mediated oxidase-like activity. ß-CD@MnO2 NNS can directly oxidizes 3,3',5,5'-tetramethylbenzidine (TMB) into oxidized TMB with color changes, yielding more than 3-fold higher oxidase-like catalytic activity compared with individual MnO2 NNS. After adding 6-MP, ß-CD@MnO2 NNS can be reduced to Mn2+ and lose their oxidase-like properties, resulting in a color and absorbance change for sensitive and selectivity detection of 6-MP. Meanwhile, the smartphone-based color recognition application can intuitively and simply measure the concentration of 6-MP. The limits of detection UV-vis instrument and smartphone were 0.35 µM and 0.86 µM, respectively. This method has also been successfully applied to the detection of real samples. Finally, this study provides a new promising platform for detection of 6-MP and is expected to be used in application of pharmaceutical analysis and biomedicine.

A colorimetric and electrochemical dual-mode Hg2+ sensor utilizing oxidase-like activity arising from the combination of Hg2+ and palladium metal-organic framework@graphene.

Developing convenient and reliable methods for Hg2+ monitoring is highly important. Some precious metal nanomaterials with intriguing peroxidase-like activity have been used for highly sensitive Hg2+ detection. However, H2O2 must be added during these detections, which impedes practical applications of Hg2+ sensors due to its susceptible decomposition by environmental factors. Herein, we discovered that the combination of Hg2+ and palladium metal-organic framework@graphene (Pd-MOF@GNs) exhibits oxidase-like activity (OXD). In the absence of H2O2, this activity not only catalyzes the oxidation of chromogenic substrates such as 3,3',5,5'-tetramethylbenzidine (TMB) or o-phenylenediamine (OPD) to produce a color change but also enhances the electrical signals during OPD oxidation. Based on these properties, an effective and convenient dual-mode colorimetric and electrochemical sensor for Hg2+ has been developed. The colorimetric and amperometric linear relationships for Hg2+ were 0.045 µM-0.25 mM and 0.020 µM-2.0 mM, respectively. The proposed strategy shows good recovery in real sample tests, indicating promising prospects for multiple environmental sample detection of Hg2+ without relying on H2O2. The colorimetric and electrochemical dual-mode Hg2+ sensor is expected to hold great potentials in applications such as environmental monitoring, rapid field detection, and integration into smartphone detection of Hg2+.

Dual-signal detection of tannic acid in red wines based on the peroxidase activity of carbon dots.

Herein, a novel type of phosphorus and iron-doped carbon dot (P,Fe-CD) with outstanding peroxidase activity and excellent fluorescence performance was hydrothermally synthesized to colorimetrically and fluorimetrically detect tannic acid (TA). In the presence of 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2, the P,Fe-CDs could oxidize colorless TMB to a blue oxidation product (oxTMB) resulting in an increased value of absorbance. Simultaneously, the fluorescence intensity of P,Fe-CDs at 430 nm could be quenched owing to the fluorescence resonance energy transfer (FRET) between P,Fe-CDs and the generated oxTMB. Meanwhile, after adding the TA to the system containing TMB, H2O2 and P,Fe-CDs, the value of absorbance could be decreased and the fluorescence could be recovered because of the reduction reaction between TA and oxTMB. Therefore, fluorescence intensity and value of absorbance could be applied to quantitatively detect TA with good linearities between the concentration of TA and the fluorescence intensity/value of absorbance (0.997 and 0.997 for the colorimetric signal and fluorimetric one, respectively) and low limits of detection (0.093 µmol L-1 and 0.053 µmol L-1 for the colorimetry and the fluorimetry, respectively), which was successfully applied to the detection of TA in red wines. Moreover, we applied a smartphone-assisted method to the point-of-care detection of TA with accurate results, providing a new technique for TA detection and food quality monitoring.

Graphene quantum dots on TiO2 nanotubes as a light-assisted peroxidase nanozyme.

Hybrid nanozyme graphene quantum dots (GQDs) deposited TiO2 nanotubes (NTs) on titanium foil (Ti/TiO2 NTs-GQDs) were manufactured by bestowing the hybrid with the advantageous porous morphology, surface valence states, high surface area, and copious active sites. The peroxidase-like activity was investigated through the catalytic oxidation of chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2, which can be visualized by the eyes. TiO2 NTs and GQDs comprising oxygen-containing functional groups can oxidize TMB in the presence of H2O2 by mimicking peroxidase enzymes. The peroxidase-mimicking activity of hybrid nanozyme was significantly escalated by introducing light illumination due to the photosensitive features of the hybrid material. The peroxidase-like activity of Ti/TiO2 NTs-GQDs enabled H2O2 determination over the linear range of 7 to 250 µM, with a LOD of 2.1 µM. The satisfying peroxidase activity is possibly due to the unimpeded access of H2O2 to the catalyst's active sites. The porous morphology provides the easy channeling of reactants and products. The periodic structure of the material also gave rise to acceptable reproducibility. Without material functionalization, the Ti/TiO2 NTs-GQDs can be a promising substitute for peroxidases for H2O2 detection.

Dual amplification for PEC ultrasensitive aptasensing of biomarker HER-2 based on Z-scheme UiO-66/CdIn2S4 heterojunction and flower-like PtPdCu nanozyme.

As an important prognostic indicator in breast cancer, human epithelial growth factor receptor-2 (HER-2) is of importance for assessing prognosis of breast cancer patients, whose accurate and facile analysis are imperative in clinical diagnosis and treatment. Herein, photoactive Z-scheme UiO-66/CdIn2S4 heterojunction was constructed by a hydrothermal method, whose optical property and photoactivity were critically investigated by a range of techniques, combined by elucidating the interfacial charge transfer mechanism. Meanwhile, PtPdCu nanoflowers (NFs) were fabricated by a simple aqueous wet-chemical method, whose peroxidase (POD)-mimicking catalytic activity was scrutinized by representative tetramethylbenzidine (TMB) oxidation in H2O2 system. Taken together, the UiO-66/CdIn2S4 based photoelectrochemical (PEC) aptasensor was established for quantitative analysis of HER-2, where the detection signals were further magnified through catalytic precipitation reaction towards 4-chloro-1-naphthol (4-CN) oxidation (assisted by the PtPdCu NFs nanozyme). The PEC aptasensor presented a broader linear range within 0.1 pg mL-1-0.1 µg mL-1 and a lower limit of detection of 0.07 pg mL-1. This work developed a new PEC aptasensor for ultrasensitive determination of HER-2, holding substantial promise for clinical diagnostics.

A papain-based colorimetric catalytic sensing system for immunoassay detection of carcinoembryonic antigen.

A colorimetric immunoassay was built for determination of carcinoembryonic antigen (CEA) based on papain-based colorimetric catalytic sensing system through the use of glucose oxidase (GOx). In the presence of GOx, glucose was catalytically oxidized to produce H2O2. Through the assistance of papain (as a peroxide mimetic enzyme), the signal came from the oxidative color development of 3,3',5,5'-tetramethylbenzidine (TMB, from colorless to blue) catalyzed by the generated H2O2. Herein, a sandwich-type immunoassay was built based on GOx as labels. As the concentration of CEA increased, more GOx-labeled antibodies specifically associate with target, which leaded to more H2O2 generation. Immediately following this, more TMB were oxidized with the addition of papain. Accordingly, the absorbance increased further. As a result, the concentration of CEA is positively correlated with the change in absorbance of the solution. Under optimal conditions, the CEA concentration was linear in the range of 0.05-20.0 ng/mL, and the limit of detection (LOD) reached 37 pg/mL. The papain-based colorimetric immunoassay also exhibited satisfactory repeatability, stability, and selectivity.

Colorimetric aptasensor based on magnetic beads and gold nanoparticles for detecting mucin 1.

In this work, a colorimetric aptasensor based on magnetic beads (MBs), gold nanoparticles (AuNPs) and Horseradish Peroxidase (HRP) was prepared for the detection of mucin 1 (MUC1). Complementary DNA of the MUC1 aptamer (Apt) immobilized on the MBs was combined with the prepared AuNPs-Apt-HRP complex (AuNPs@Apt-HRP). In the presence of MUC1, it specifically bound to Apt, resulting in the detachment of gold nanoparticles from the MBs. After magnetic separation, AuNPs@Apt-HRP was separated into the supernatant and reacted with 3,3',5,5'-Tetramethylbenzidine (TMB) to produce color reaction from colorless to blue. The linear range of MUC1 was from 75 to 500 µg/mL (R2 = 0.9878), and the detection limit was 41.95 µg/mL. The recovery rate of MUC1 in human serum was 99.18 %∼101.15 %. This method is simple and convenient. Moreover, it does not require complex and expensive equipment for detection of MUC1. It provides value for the development of MUC1 colorimetric sensors and a promising strategy for the determination of MUC1 in clinical diagnosis.