A colorimetric test for the evaluation of the insecticide content of LLINs used on Bioko Island, Equatorial Guinea.

BACKGROUND: Insecticide-treated nets and indoor residual spraying of insecticides are used as the vector control interventions in the fight against malaria. Measuring the actual amount of deposits of insecticides on bed nets and walls is essential for evaluating the quality and effectiveness of the intervention. A colorimetric "Test Kit" designed for use as a screening tool, able to detect the type II pyrethroids on fabrics and sprayed walls, was used for the first time to detect deltamethrin on long-lasting insecticidal nets (LLINs) deployed on Bioko Island, Equatorial Guinea. METHODS: LLINs were analysed using the colorimetric Test Kit performed in situ, which leads to the formation of an orange-red solution whose depth of colour indicates the amount of type II pyrethroid on the net. The kit results were validated by measuring the amount of extracted insecticide using high-performance liquid chromatography (HPLC) with diode array detection (DAD). RESULTS: Deltamethrin concentration was determined for 130 LLINs by HPLC-DAD. The deltamethrin concentration of these nets exhibited a significant decrease with the age of the net from 65 mg/m2 (< 12 months of use) to 31 mg/m2 (> 48 months; p < 0.001). Overall, 18% of the nets being used in households had < 15 mg/m2 of deltamethrin, thus falling into the "Fail" category as assessed by the colorimetric Test Kit. This was supported by determining the bio-efficacy of the nets using the WHO recommended cone bioassays. The Test Kit was field evaluated in situ and found to be rapid, accurate, and easy to use by people without laboratory training. The Test Kit was shown to have a reliable linear relationship between the depth of colour produced and deltamethrin concentration (R2 = 0.9135). CONCLUSION: This study shows that this colorimetric test was a reliable method to assess the insecticidal content of LLINs under operational conditions. The Test Kit provides immediate results and offers a rapid, inexpensive, field-friendly alternative to the complicated and costly methods such as HPLC and WHO cone bioassays which also need specialist staff. Thus, enabling National Malaria Control Programmes to gain access to effective and affordable monitoring tools for use in situ.

Tumor Biomarker In-Solution Quantification, Standard Production, and Multiplex Detection.

The diagnosis and monitoring of cancer have been facilitated by discovering tumor "biomarkers" and methods to detect their presence. Yet, for certain cancers, we still lack sensitive and specific biomarkers or the means to quantify subtle concentration changes successfully. The identification of new biomarkers of disease and improving the sensitivity of detection will remain key to changing clinical outcomes. Patient liquid biopsies (serum and plasma) are the most easily obtained sources for noninvasive analysis of proteins that tumor cells release directly and via extracellular microvesicles and tumor shedding. Therefore, an emphasis on creating reliable assays using serum/plasma and "direct, in-solution" ELISA approaches has built an industry centered on patient protein biomarker analysis. A need for improved dynamic range and automation has resulted in the application of ELISA principles to paramagnetic beads with chemiluminescent or fluorescent detection. In the clinical testing lab, chemiluminescent paramagnetic assays are run on automated machines that test a single analyte, minimize technical variation, and are not limited by serum sample volumes. This differs slightly from the R&D setting, where serum samples are often limiting; therefore, multiplexing antibodies to test multiple biomarkers in low serum volumes may be preferred. This review summarizes the development of historical biomarker "standards", paramagnetic particle assay principles, chemiluminescent or fluorescent biomarker detection advancements, and multiplexing for sensitive detection of novel serum biomarkers.

Colorimetric Reverse Transcription-Loop-Mediated Isothermal Amplification Assay for Rapid Detection of SARS-CoV-2.

Coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been spreading rapidly all over the world. In the absence of effective treatments or a vaccine, there is an urgent need to develop a more rapid and simple detection technology of COVID-19. We describe a WarmStart colorimetric reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay for the detection of SARS-CoV-2. The detection limit for this assay was 1 copy/µL SARS-CoV-2. To test the clinical sensitivity and specificity of the assay, 37 positive and 20 negative samples were used. The WarmStart colorimetric RT-LAMP had 100% sensitivity and specificity. End products were detected by direct observation, thereby eliminating the need for post-amplification processing steps. WarmStart colorimetric RT-LAMP provides an opportunity to facilitate virus detection in resource-limited settings without a sophisticated diagnostic infrastructure.

Optimization of a Colorimetric Assay to Determine Lactate Dehydrogenase B Activity Using Design of Experiments.

Lactate dehydrogenase B (LDH-B) is overexpressed in lung and breast cancer, and it has been considered as a potential target to treat these types of cancer. Herein, we propose a straightforward incomplete factorial (IF) design composed of 12 combinations of two reaction buffers, three pH values, three salt (NaCl) concentrations, and three incubation times, which we called IF-BPST (Buffer/pH/Salt/Time), for the optimization of a colorimetric LDH-B assay in a final volume of 100 µL using 96-well plates. The assay is based on the absorbance change at ~570 nm and the color change of the reaction mixture due to the release of NADH that reacts with nitroblue tetrazolium (NBT) and phenazine methosulfate (PMS), resulting in the formation of a blue-purple formazan. The results obtained using the IF-BPST were comparable with those obtained by response surface methodology. Our work revealed that the NBT/PMS assay with some modifications can be used to measure the activity of LDH-B and other dehydrogenases in a high-throughput screening format at the early stages of drug discovery. LDH-B containing lysates cannot be assayed directly, however, due to the sensitivity of the method toward detergents. Thus, we suggest precipitating the proteins in the lysates to remove the interfering detergents, and then to dissolve the protein pellet in a suitable buffer and carry out the assay.

Disposable and Low-Cost Colorimetric Sensors for Environmental Analysis.

Environmental contamination affects human health and reduces the quality of life. Therefore, the monitoring of water and air quality is important, ensuring that all areas are acquiescent with the current legislation. Colorimetric sensors deliver quick, naked-eye detection, low-cost, and adequate determination of environmental analytes. In particular, disposable sensors are cheap and easy-to-use devices for single-shot measurements. Due to increasing requests for in situ analysis or resource-limited zones, disposable sensors' development has increased. This review provides a brief insight into low-cost and disposable colorimetric sensors currently used for environmental analysis. The advantages and disadvantages of different colorimetric devices for environmental analysis are discussed.

Development of standardized method for the quantification of azo dyes by UV-Vis in binary mixtures.

The azo dyes, Yellow 5 (Y5), Red 2 (R2) and Blue 1 (B1), quantified in solutions and in mixtures of binary dyes, were studied by means of UV-Vis spectroscopy. In this work was used a CIE algorithm developed in Visual Basic for Applications (VBA). The CIE algorithm is based on the tristimulus chromaticity diagram, as an alternative to the shielding effect that arises in dye mixtures, and it can also be applied to complex quantification methods such as HPLC (High Performance Liquid Chromatography). The results obtained through of the algorithm, showed a higher accuracy from 97 to 99% in relation with similar UV-Vis quantification methods. In contrast, linear methods only managed to reach an accuracy from 78 to 98%. Additionally, the algorithm yielded significant similar values to the UHPLC reference method. The results showed that the method CIE algorithm was accessible and reliable to quantify binary mixtures of the dyes used which suggests the possibility to apply this method on other dyes, within the limits of quantification obtained in this study (0.076-24.56 mg/L) and the pH values from 2 to 10.

Development and validation of an economical uric acid-Fe3+/Fe2+-ferrozine-based colorimetric assay to estimate uric acid level of pure and biological samples.

This work presents the development and validation of a simple, rapid, and cost-effective spectrophotometric method for quantitative analysis of uric acid in biological samples. The method relies upon uric acid-led reduction of Fe(III) to Fe(II) of sample/standard solutions which stoichiometrically engages ferrozine to form a magenta-colored complex. Different parameters including pH, metal and chelator concentrations, temperature, etc., were optimized for the maximum intensity and stability of the complex. The uric acid concentrations of synthetic/plasma solutions were determined by comparing the color intensity of Fe(ferrozine)3 2+ complex produced by test solution with the standard curve formed by known uric acid concentrations. The method was validated in accordance with ICH guidelines and subjected to human plasma analysis. The results obtained were compared with a reference (enzymatic) method which revealed that there was no significant difference between the two methods at 95% confidence level. The method is highly specific, precise, linear, accurate, and robust.

A Microdroplet-Based Colorimetric Sensing Platform on a CMOS Imager Chip.

Interest in mobile chemical sensors is on the rise, but significant challenges have restricted widespread adoption into commercial devices. To be useful these sensors need to have a predictable response, easy calibration, and be integrable with existing technology, preferably fitting on a single chip. With respect to integration, the CMOS imager makes an attractive template for an optoelectronic sensing platform. Demand for smartphones with cameras has driven down the price and size of CMOS imagers over the past decade. The low cost and accessibility of these powerful tools motivated us to print chemical sensing elements directly on the surface of the photodiode array. These printed colorimetric microdroplets are composed of a nonvolatile solvent so they remain in a uniform and homogeneous solution phase, an ideal medium for chemical interactions and optical measurements. By imaging microdroplets on the CMOS imager surface we eliminated the need for lenses, dramatically scaling down the size of the sensing platform to a single chip. We believe the technique is generalizable to many colorimetric formulations, and as an example we detected gaseous ammonia with Cu(II). Limits of detection as low as 27 ppb and sensor-to-sensor variation of less than 10% across multiple printed arrays demonstrated the high sensitivity and repeatability of this approach. Sensors generated this way could share a single calibration, greatly reducing the complexity of incorporating chemical sensors into mobile devices. Additional testing showed the sensor can be reused and has good selectivity; sensitivity and dynamic range can be tuned by controlling droplet size.

Evaluation of SARS-CoV-2 neutralizing antibodies using a CPE-based colorimetric live virus micro-neutralization assay in human serum samples.

The micro-neutralization assay is a fundamental test in virology, immunology, vaccine assessment, and epidemiology studies. Since the SARS-CoV-2 outbreak at the end of December 2019 in China, it has become extremely important to have well-established and validated diagnostic and serological assays for this new emerging virus. Here, we present a micro-neutralization assay with the use of SARS-CoV-2 wild type virus with two different methods of read-out. We evaluated the performance of this assay using human serum samples taken from an Italian seroepidemiological study being performed at the University of Siena, along with the human monoclonal antibody CR3022 and some iper-immune animal serum samples against Influenza and Adenovirus strains. The same panel of human samples have been previously tested in enzyme-linked immunosorbent assay (ELISA) as a pre-screening. Positive, borderline, and negative ELISA samples were evaluated in neutralization assay using two different methods of read-out: subjective (by means of an inverted optical microscope) and objective (by means of a spectrophotometer). Our findings suggest that at least 50% of positive ELISA samples are positive in neutralization as well, and that method is able to quantify different antibody concentrations in a specific manner. Taken together, our results confirm that the colorimetric cytopathic effect-based microneutralization assay could be used as a valid clinical test method for epidemiological and vaccine studies.

Rapid colorimetric loop-mediated isothermal amplification for hypersensitive point-of-care Staphylococcus aureus enterotoxin A gene detection in milk and pork products.

Staphylococcus aureus strains carrying enterotoxin A gene (sea) causes food poisoning and cannot be distinguished from non-pathogenic strains by the culture method. Here, we developed a rapid, specific and sensitive visual detection of sea using loop-mediated isothermal amplification (LAMP) combined with nanogold probe (AuNP) or styryl dye (STR). LAMP-AuNP and LAMP-STR can detect as low as 9.7 fg (3.2 sea copies) and 7.2 sea copies, respectively, which were lower than PCR (97 fg or 32 sea copies). The excellent performance of these new assays was demonstrated in food samples using crude DNA lysates. While the culture method detected 104 CFU/g in ground pork and 10 CFU/mL in milk in 5-7 days, LAMP-AuNP could detect down to 10 CFU/g for both samples in 27 minutes. Analyzing 80 pork and milk samples revealed that the LAMP-AuNP showed 100% sensitivity, 97-100% specificity and 97.5-100% accuracy, which were superior to the culture method, and comparable to PCR but without requirement of a thermal cycler. Furthermore, our LAMP-AuNP detect sea at a range below the food safety control (<100 CFU/g). The LAMP-STR quantitated sea in 10-1,000 CFU (7.2-720 copies). Our crude DNA lysis combined with LAMP-AuNP/STR present effective point-of-care detection and facilitate appropriate control strategies.

Hemin@carbon dot hybrid nanozymes with peroxidase mimicking properties for dual (colorimetric and fluorometric) sensing of hydrogen peroxide, glucose and xanthine.

The multifunctional hemin@carbon dot hybrid nanozymes (hemin@CD) with simultaneous peroxidase-like activity and fluorescence signalling property was prepared for the first time. Based on these properties, hemin@CD was applied to develop a dual-channel fluorescent probe for H2O2 and H2O2-based biocatalytic systems. By virtue of the peroxidase-like activity, hemin@CD can catalyze the oxidative coupling of 4-aminoantipyrine with phenol in the presence of H2O2 to form a pink-red quinoneimine dye with a maximum absorbance at 505 nm. Under the excitation wavelength of 480 nm, the green fluorescence of hemin@CD peaks at 540 nm and is quenched by the generated quinoneimine dye due to an inner filter effect, and also by H2O2 because of dynamic quenching. Thus, a colorimetric and fluorimetric dual-channel optical probe for H2O2 is obtained. Due to the glucose/xanthine transformations under formation of H2O2 by the relevant oxidase catalysis, the probe can be applied for detection of glucose and xanthine. The colorimetric detection limits for H2O2, glucose and xanthine are 0.11, 0.15, 0.11 µM, and the and fluorimetric detection limits are 0.15, 0.15, 0.12 µM, respectively. Graphical abstractSchematic representation of the colorimetric and fluorimetric dual probe for H2O2, glucose and xanthine based on the multifunctional emin@carbon dot) hybrid nanozymes with simultaneous peroxidase-like activity and fluorescence signalling property.

Smart Chip for Visual Detection of Bacteria Using the Electrochromic Properties of Polyaniline.

Finding fast and reliable ways to detect pathogenic bacteria is crucial for addressing serious public health issues in clinical, environmental, and food settings. Here, we present a novel assay based on the conversion of an electrochemical signal into a more convenient optical readout for the visual detection of Escherichia coli. Electropolymerizing polyaniline (PANI) on an indium tin oxide screen-printed electrode (ITO SPE), we achieved not only the desired electrochromic behavior but also a convenient way to modify the electrode surface with antibodies (taking advantage of the many amine groups of PANI). Applying a constant potential to the PANI-modified ITO SPE induces a change in their oxidation state, which in turn generates a color change on the electrode surface. The presence of E. coli on the electrode surface increases the resistance in the circuit affecting the PANI oxidation states, producing a different electrochromic response. Using this electrochromic sensor, we could measure concentrations of E. coli spanning 4 orders of magnitude with a limit of detection of 102 colony forming unit per 1 mL (CFU mL-1) by the naked eye and 101 CFU mL-1 using ImageJ software. In this work we show that merging the sensitivity of electrochemistry with the user-friendliness of an optical readout can generate a new and powerful class of biosensors, with potentially unlimited applications in a variety of fields.

Highly Selective, Aptamer-Based, Ultrasensitive Nanogold Colorimetric Smartphone Readout for Detection of Cd(II).

A highly selective and sensitive method for Cd(II) detection was developed based on aptamer and gold nanoparticles (AuNPs) combined with a colorimetric smartphone readout. The experimental conditions such as reaction time of polydiene dimethyl ammonium chloride (PDDA) and AuNPs, PDDA dose, time of aptamer and PDDA incubation, and aptamer concentration were optimized. Under the optimized conditions, the color and red(R) value of the solution was concentration-dependent on Cd(II). The proposed method exhibited a linear range of 1-400 ng/mL (r2 = 0.9794) with a limit of detection (LOD) of 1 ng/mL. This method had been successfully applied to test and quantify Cd(II) in water and rice samples, and the results were in full agreement with those from the atomic absorption spectrometer. Therefore, low-cost colorimetry demonstrated its potential for practical application in visual or quantitative detection with a smartphone. This approach can be readily applied to other analytes.

Synergistically enhanced colorimetric molecular detection using smart cup: a case for instrument-free HPV-associated cancer screening.

Rationale: Early and accurate detection of disease is crucial for its prevention, identification, and treatment. However, most of disease diagnostics is still limited in clinical laboratories due to the need of complicated instruments and professional personnel. Herein, we reported a smartphone-based synergistically enhanced colorimetric method for molecular diagnostics in our point of care (POC) smart cup platform. Methods: A disposable microfluidic chip was developed for colorimetric loop-mediated isothermal amplification (LAMP) detection of multiple HPV DNA in our POC smart cup platform. The colorimetric detection takes advantage of synergistic effect of PPi4- and H+ ions, two byproducts of LAMP reaction. Color signal of LAMP assay was recorded and analyzed by our custom Android app (dubbed "Hue Analyzer"). Results: Our method not only significantly improves colorimetric readout, but also provides a 10-fold increase in detection sensitivity. It has been successfully applied for HPV-associated cancer screening with spiked saliva and clinical swab samples. Conclusion: The proposed POC diagnostic platform is completely compatible with other nucleic acid biomarkers and has great potential for personalized health monitoring and disease prevention.

Enhanced synergistic effects from multiple iron oxide nanoparticles encapsulated within nitrogen-doped carbon nanocages for simple and label-free visual detection of blood glucose.

Hollow-structured carbon materials play a crucial role in research of biosensors, energy storage and nanomedicine as a kind of material with advantages like high surface area, tunable pore volume, excellent mechanical properties, and good biocompatibility. Herein, we developed a simple, facile and controllable method for synthesis of Fe3O4 nanoparticles encapsulated in hollow carbon nanocages (FNHCs) with SiO2 nanospheres as a sacrificial template. Owing to the unique structure of multiple Fe3O4 nanoparticles cores integrated with N-doped carbon nanocages, the as-synthesized FNHCs exhibited greatly enhanced peroxidase mimicking activity with extremely high signal-to-noise ratio of ∼91 fold. Also, it was found that the FNHCs possessed a higher peroxidase-like activity than that of other similar-structured Fe3O4 architectures (e.g. Fe3O4@C NPs). The resulting steady-state kinetic curve demonstrated the enzymatic activity of FNHCs with classic Michaelis-Menton kinetics following a ping-pong mechanism. On the basis of the superior enzymatic activity, the FNHCs performed as a high-efficiency peroxidase mimic, realizing facile, label-free, highly sensitive/selective colorimetric detection of H2O2 and glucose. Furthermore, the colorimetric sensor successfully determined glucose in patients' serum samples with high accuracy and precision, suggesting great potential for real applications.

A hairpin-type DNA probe for direct colorimetric detection of endonuclease activity and inhibition based on the deaggregation of gold nanoparticles.

A method is described for the determination of the activity of endonuclease. It based on the deaggregation of gold nanoparticles (AuNPs) aggregated by the action of poly(diallyldimethylammonium chloride) (PDDA). A single-stranded DNA (ssDNA) is released after enzymatic cleavage catalyzed by endonuclease. The released fragments bind electrostatically to PDDA and inhibit the PDDA-induced aggregation of AuNPs. This is accompanied by a color change from blue to red and a decrease in the absorption ratio (A630/A520). Under the optimal conditions, this ratio increases linearly in the 0.001 to 1 U·µL-1 EcoRI endonuclease activity range. The detection limit is of 2 × 10-4 U·µL-1 which is much better or at least comparable to previous reports. The method is deemed to have wide scope in that it may be used to study other endonuclease activity (such as BamHI) by simply changing the specific recognition site of the hairpin-like DNA probe. The assay may also be employed to screening for inhibitors of EcoRI endonuclease. Graphical abstract Schematic presentation of the colorimetric assay based on the deaggregation of AuNPs for the detection of endonuclease activity. A single-stranded sequence (ssDNA) is released by the EcoRI cleavage, which electrostatically binds to PDDA and inhibits the PDDA-induced aggregation of AuNPs accompanying with a color change from blue to red.

MODS-Wayne, a Colorimetric Adaptation of the Microscopic-Observation Drug Susceptibility (MODS) Assay for Detection of Mycobacterium tuberculosis Pyrazinamide Resistance from Sputum Samples.

Although pyrazinamide (PZA) is a key component of first- and second-line tuberculosis treatment regimens, there is no gold standard to determine PZA resistance. Approximately 50% of multidrug-resistant tuberculosis (MDR-TB) and over 90% of extensively drug-resistant tuberculosis (XDR-TB) strains are also PZA resistant. pncA sequencing is the endorsed test to evaluate PZA susceptibility. However, molecular methods have limitations for their wide application. In this study, we standardized and evaluated a new method, MODS-Wayne, to determine PZA resistance. MODS-Wayne is based on the detection of pyrazinoic acid, the hydrolysis product of PZA, directly in the supernatant of sputum cultures by detecting a color change following the addition of 10% ferrous ammonium sulfate. Using a PZA concentration of 800 µg/ml, sensitivity and specificity were evaluated at three different periods of incubation (reading 1, reading 2, and reading 3) using a composite reference standard (MGIT-PZA, pncA sequencing, and the classic Wayne test). MODS-Wayne was able to detect PZA resistance, with a sensitivity and specificity of 92.7% and 99.3%, respectively, at reading 3. MODS-Wayne had an agreement of 93.8% and a kappa index of 0.79 compared to the classic Wayne test, an agreement of 95.3% and kappa index of 0.86 compared to MGIT-PZA, and an agreement of 96.9% and kappa index of 0.90 compared to pncA sequencing. In conclusion, MODS-Wayne is a simple, fast, accurate, and inexpensive approach to detect PZA resistance, making this an attractive assay especially for low-resource countries, where TB is a major public health problem.

Sunlight caused interference in outdoor N, N-diethyl-p-phenylenediamine colorimetric measurement for residual chlorine and the solution for on-site work.

Chlorination is the most common method to control water qualities, in some case on-site outdoor measurements are required to measure easily-decaying residual chlorine concentration appropriately without delay. In this study sunlight-induced unexpected colour development (UCD) of N, N-diethyl-p-phenylenediamine (DPD) colorimetric measurement was studied under several sun exposure conditions. The colour development level was evaluated with reference to chlorine concentration (mg/L) and relationships between colour development rate (mg/L min) and intensities of solar were investigated. UCD was found to be related to both exposure intensity and time. By means of exposure experiment under specific wavelength of ultraviolet (UV), it was confirmed that both middle and short wavelength of UV radiation being responsible for such an unexpected measurement. Consequently, a simple device was designed using three commercially available anti-UV films, one of which could effectively prevent the UCD from direct sun exposure.

Study of Simple Immunohistochemical Cytocolorimetric Assay Application for More Accurate Assessment of Prognosis in Patients with Pituitary Adenomas.

BACKGROUND: Immunohistochemistry is a basic diagnostic technique. Immunohistochemical examination results reflect mainly qualitative and less quantitative characteristics of proteomic status of cells. A combined approach with complex quantitative evaluation of marker expression using colorimetric analysis and computer technologies can expand the diagnostic capabilities of immunohistochemistry. We studied such an approach developed by using expression of the proliferative marker Ki-67 in pituitary adenomas. METHODS: A retrospective, blind, randomized, comparative study was performed of Ki-67 expression activity in pituitary adenomas using the traditional Ki-67 labeling index and a simple immunohistochemical cytocolorimetric analysis developed by us with immunohistochemical cytocolorimetric index (ICI) estimation as predictors of relapse, assessing the relationships of these indicators with the time before relapse. RESULTS: Mean Ki-67 labeling index was 3.87% ± 0.29% in the relapse-free group and 4.01% ± 0.29% in the relapse group; the difference was not statistically significant. The average Ki-67 ICI was 24.16% ± 0.51% in the relapse-free group and 30.68% ± 0.64% in the relapse group; the difference was statistically significant. The correlation coefficient of ICI values and time before relapse was -0.302, indicating the presence of a weak negative correlation. CONCLUSIONS: We successfully tested an ICI estimation method developed by us to assess Ki-67 expression in pituitary adenomas. The ICI technique can be used both as a prognostic factor for relapse and, in combination with other modern proteomic and genetic methods, as the basis for creation of new multimodal analyzing systems for functional state assessment of cells and tissues.

A colorimetric mercury(II) assay based on the Hg(II)-stimulated peroxidase mimicking activity of a nanocomposite prepared from graphitic carbon nitride and gold nanoparticles.

A one-step reduction method was used for the preparation of stable graphitic carbon nitride-gold nanoparticles (g-C3N4-Au) nanocomposites from ultrathin g-C3N4 nanosheets and chloroauric acid by using NaBH4 as a reducing agent under ultrasonication. The nanocomposites were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis absorption and fluorescence spectroscopy etc. The results revealed that the gold nanoparticles (AuNPs) are uniformly formed on the g-C3N4 nanosheets. It is found that the peroxidase-like catalytic activity of this nanocomposite for the oxidation of 3,3',5,5'-tetramethylbenzidine by H2O2 to form a blue-colored product is strongly enhanced in the presence of Hg(II). Based on this phenomenon, a sensitive "turn-on" colorimetric assay for Hg(II) was developed that works at physiological pH values. Under optimal conditions, the absorption signal at 652 nm increases linearly with Hg(II) concentration in the range from 5 to 500 nM. A detection limit as low as 3.0 nM was achieved. This assay has excellent selectivity over other metal ions. It was successfully applied to the determination of Hg(II) in real water samples. The method is cost-effective, rapid, and allows for visual detection. Graphical abstract The nanocomposite composed of graphitic carbon nitride (g-C3N4) and gold nanoparticles (g-C3N4-AuNPs) can catalyze tetramethylbenzidine (TMB) oxidation by H2O2 to produce light-blue product (oxTMB). The peroxidase-like activity of g-C3N4-AuNPs can be greatly enhanced by Hg2+, thus increases the amount of the blue product formed.