In recent years, loop-mediated isothermal amplification (LAMP) has gained popularity for detecting various pathogen-specific genes due to its superior sensitivity and specificity compared to conventional polymerase chain reaction (PCR). The simplicity and flexibility of naked-eye detection of the amplicon make LAMP an ideal rapid and straightforward diagnostic tool, especially in resource-limited laboratories. Colorimetric detection is one of the simplest and most straightforward among all detection methods. This review will explore various colorimetric dyes used in LAMP techniques, examining their reaction mechanisms, advantages, limitations and latest applications.

The monitoring of foodborne bacterial contamination requires simple and convenient biosensors. This work describes a novel paper-based colorimetric biosensor for the rapid and sensitive bacteria detection. The biosensor was constructed via the encapsulation of D-alanyl-D-alanine capped gold nanoparticles (DADA-AuNPs) in a modified paper that was fabricated by the freeze-drying of TEMPO-oxidized cellulose nanofibers/cationic guar gum composite hydrogel-modified filter paper. The results indicated that the size of DADA-AuNPs largely determined the color of their aqueous system and they exhibited light red to dark red as their size increased from around 6 to 36 nm. All these different sized DADA-AuNPs turned into colorless when encountered with either S. aureus or E. coli. In particular, the smaller the DADA-AuNPs size, the faster the discoloration. The encapsulation of DADA-AuNPs into modified paper negligibly changed their responsiveness towards bacteria. In comparison to the original filter paper and oven-dried hydrogel-modified filter paper, the freeze-dried hydrogel-modified paper was demonstrated to be a better substrate for the encapsulation of DADA-AuNPs since they could be loaded with a larger amount of DADA-AuNPs in a faster way and showed a better perceivable color. This work demonstrated a promising paper-based colorimetric biosensor for the facile and rapid detection of bacteria.

The presence of nitrite in food products has generated significant public concern. A simple and rapid dual-mode surface-enhanced Raman spectroscopy (SERS)/colorimetric detection of nitrite is proposed based on a diazo reaction and multifunctional gold nanoparticle-doped covalent organic framework (Au@COF) composite. Under acidic conditions, the reaction between toluidine blue and nitrite yielded a colorless diazo salt, simultaneously attenuating its characteristic absorption peak and Raman signal. The multifunctional Au@COF materials enhanced the Raman signal and ensured good reproducibility. Additionally, the reaction rates improved, and the sensitivity was enhanced due to the excellent adsorption capacity of the COF. The proposed method demonstrated high sensitivity and excellent recovery rates for nitrite detection in food samples. This approach shows potential for precisely detecting nitrite content in real-world food samples by integrating the simplicity of colorimetric analysis with the enhanced sensitivity of SERS.

A newly synthesized naphthalimide-based fluorophore probe NIA was used to detect hydrazine. This probe, based on the Gabriel mechanism exhibited a highly sensitive revealing of hydrazine in naked eyes colorimetric as well as fluorescent recognition against other amines in an aqueous solution in DMSO - HEPES buffer. When hydrazine hydrate was added to the probe NIA, the absorption was red shifted from 403 nm to 520 nm. The titration studies by adding hydrazine to show two apparent isosbestic points found at 358 and 450 nm, respectively. Further, investigation of emission spectra upon addition of hydrazine hydride the emission peak at 493 nm gradually decreased up to 2.4 equiv. and when increasing the hydrazine hydride concentration from 2.4 equiv. to 4.4 equiv., the fluorescence intensity increased at 530 nm. which is exhibiting a raised ratiometric emission intensity at 530 nm. Further investigation of the selectivity of probe NIA revealed colorimetric and fluorimetric responses to interferences with other test amines. 1H NMR and HR-mass proved the Gabriel mechanism bath for detecting hazardous hydrazine by probe NIA. This probe NIA allowed the rapid and ultrasensitive detection of hydrazine hydride with a low detection limit of 0.26 nM. In view of the outstanding properties, probe NIA has been effectively performed to detect hydrazine using various techniques, including a test kit, silica support, and actual environmental water samples.

A ratiometric fluorescent and colorimetric detecting assay for NO2- was realized by a hybrid nanosensor (Co2+-CDs@R-CDs) utilizing firstly through the redox reaction of nitrite (NO2-) with Co2+, of which the hybrid nanosensor Co2+-CDs@R-CDs was fabricated by Co2+-doped carbon dots (Co2+-CDs) and a reference of red-emitting carbon dots (R-CDs). The ratiometric fluorescent linear detection range of NO2- was 2.5-45 μM and the limit of detection (LOD) was 0.068 μM with the response time of 120 s. While, the colorimetric linear detection range of NO2- was 2.5-60 μM and the LOD was 0.075 μM. In addition, a portable smartphone system which could measure the R (red), G (green), and B (blue) values of the fluorescence and the visible color of the coated Co2+-CDs@R-CDs paper strip-based sensor had also been developed and successfully applied to detect NO2- in sausage, pickles and tap water samples.

RT-LAMP is an effective alternative to RT-PCR-based diagnostics, offering high specificity, sensitivity, and rapid results. One notable advantage is the robustness of its enzymes, allowing for direct amplification from crude samples without the need for prior isolation of RNA. Colorimetric LAMP is particularly attractive as it eliminates the need for complex instrumentation, making it suitable for point-of-care applications. Here, we present a comprehensive step-by-step protocol for establishing an RT-LAMP-based test for direct detection of SARS-CoV-2 genomic RNA in saliva samples using different colorimetric detection methods. Importantly, this versatile test can be easily adapted to detect emerging pathogens.

This work reports the development of low-cost and rapid multiplexed colorimetric assay of antioxidants (total phenolics, antioxidant capacity, flavonoids and anthocyanins) in wines at daisy-shaped fluidic paper-based analytical devices (PADs). The desired fluidic patterns were formed on paper by pen drawing and colorimetric reagents were immobilized at the 6 peripheral test zones. The sample was added at the central sample zone, migrated to the test zones and reacted with the immobilized reagents producing characteristic colors that were captured and analyzed. The paper-based approach was applied to the analysis of several wine samples and the results were statistically correlated to standard solution-based colorimetric assays, indicating that it could be reliably used for ranking wines according to their antioxidants content. In addition, the paper-based analytical methodology is simple, instrument-free, portable, cost-effective, rapid and environment friendly.

Among the foremost persistent heavy metal ions in the ecosystem, mercury (Hg2+) remains intimidating to the environment by producing a catastrophic effect on the environment as well as on mankind due to the exacerbation of anthropogenic activities. Therefore, it has become necessary to develop superlative techniques for its detection even at low concentrations. The conventional approaches for Hg2+ ions are quite laborious, and expensive, and require expertise in operating sophisticated instruments. To overcome these limitations, aptamer-based biosensors emerged as a promising tool for its detection. DNA-based aptamers have evolved as a significant technique by detecting them even in ppb levels. This review outlines the progress in aptamer-based biosensors from the year 2019-2023 by inducing changes in the electrochemical signal or by fluorescent/colorimetric approaches. The electrochemical sensors used nanomaterial electrodes for increasing the sensitivity whereas fluorescent and colorimetric sensors exhibit quenching or strong fluorescence in the presence of Hg2+ ions depending upon the prevailing mechanism or visible color changes. This perturbation in the signals could be attributed to the formation of the T-Hg2+ -T complex with the aptamers in the presence of ions revealing its real-time and biological applications in living or cancerous cells. Furthermore, next-generation biosensors are suggested to bring a paradigm shift to the integration of high-end smartphones, machine learning, artificial intelligence, etc.

Chlorpyrifos (CPF) residues in food pose a serious threat to ecosystems and human health. Herein, we propose a three-dimensional folded paper-based microfluidic analysis device (3D-μPAD) based on multifunctional metal-organic frameworks, which can achieve rapid quantitative detection of CPF by fluorescence-colorimetric dual-mode readout. Upconversion nanomaterials were first coupled with a bimetal organic framework possessing peroxidase activity to create a fluorescence-quenched nanoprobe. After that, the 3D-μPAD was finished by loading the nanoprobe onto the paper-based detection zone and spraying it with a color-developing solution. With CPF present, the fluorescence intensity of the detection zone gradually recovers, the color changes from colorless to blue. This showed a good linear relationship with the concentration of CPF, and the limits of detection were 0.028 (fluorescence) and 0.043 (colorimetric) ng/mL, respectively. Moreover, the 3D-μPAD was well applied in detecting real samples with no significant difference compared with the high-performance liquid chromatography method. We believe it has huge potential for application in the on-site detection of food hazardous substance residues.

In this work, we present a novel colorimetric sensing platform for the sensitive detection of ethamsylate (ETM) usingultrathin MnO2 nanosheets with enhancedoxidase-mimicking activity. A facile template-free hydrothermal process was applied to synthesize the MnO2 nanosheets under mild conditions. The nanosheets exhibited oxidase-mimicking activity, facilitating the conversion of TMB into the blue-colored oxTMB in the absence of H2O2. However, the presence of ETM inhibited this activity, resulting in the conversion of oxTMB back to colorless TMB and a substantial decrease in the blue color intensity. The colorimetric response exhibited a linear relationship with ETM concentration over the range of 0.5 to 10.0 µg/mL and a detection limit of 0.156 µg/mL. To further elucidate the underlying mechanism, we performed extensive characterization and kinetic experiments. The findings demonstrated that this unique property is attributed to the remarkable capacity of the MnO2 nanosheets to absorb oxygen, producing superoxide radicals (O2-). The oxidase-mimicking activity of the nanosheets was further confirmed by the reaction kinetics, following Michaelis-Menten\'s behavior. Moreover, the applicability of the sensing platform was assessed by determining ETM concentrations in various real samples (different pharmaceuticals, human plasma, and environmental water). The well-established platform demonstrates the prospective role that nanomaterials-based sensing platforms may play in clinical diagnostics, pharmaceutical analysis, and other relevant fields.