A Colorimetric Sensor for the Visual Detection of Azodicarbonamide in Flour Based on Azodicarbonamide-Induced Anti-Aggregation of Gold Nanoparticles

A ratiometric fluorescence sensor based on the inner filtration effect of gold nanoparticles on quantum dots for monitoring dopamine

In this study, a smart phone assisted ratiometric fluorescence sensor was designed for detecting dopamine (DA). The ratiometric fluorescence sensor was prepared by simple physical mixing green quantum dots (GQDS) and red quantum dots (RQDS). DA could induce gold nanoparticles (AuNPs) aggregate via hydrogen-bonding interactions, and further changed the absorption spectrum of gold nanoparticles to overlap with a certain emission spectrum of ratiometric fluorescence sensor. AuNPs had inner filtration effect (IFE) on the ratiometric fluorescence sensor. Due to the IFE, the dispersive AuNPs could quench GQDS, whereas the clustered AuNPs could quench RQDS. With the addition of DA, the color of ratiometric fluorescence changed from orange red to green. To simplify the detection process, a smartphone was employed to detecting DA in human urine by measuring RGB value of fluorescence color changes with a detection limit of 86 nM. This proposed method has the advantages of low cost, easy prevalence and simple operation, thus provides a great promise for rapid detection of biomarker in biological samples.

Progress in the design of portable colorimetric chemical sensing devices

The need for precise determination of heavy metals, anions, biomolecules, pesticides, drugs, and other substances is vital across clinical, environmental, and food safety domains. Recent years have seen significant progress in portable colorimetric chemical sensing devices, revolutionizing on-the-spot analysis. This review offers a comprehensive overview of these advancements, covering handheld colorimetry, RGB-based colorimetry, paper-based colorimetry, and wearable colorimetry devices. It explores the underlying principles, functional materials (chromophoric reagents/dyes and nanoparticles), detection mechanisms, and their applications in environmental monitoring, clinical care, and food safety. Noble metal nanoparticles (NPs) have arisen as promising substitutes in the realm of sensing materials. They display notable advantages, including heightened sensitivity, the ability to fine-tune their plasmonic characteristics for improved selectivity, and the capacity to induce visible color changes, and simplifying detection. Integration of NPs fabricated paper device with smartphones and wearables facilitates reagent-free, cost-effective, and portable colorimetric sensing, enabling real-time analysis and remote monitoring.

Plasmonic nanoparticle's anti-aggregation application in sensor development for water and wastewater analysis

Colorimetric sensors have emerged as a powerful tool in the detection of water pollutants. Plasmonic nanoparticles use localized surface plasmon resonance (LSPR)-based colorimetric sensing. LSPR-based sensing can be accomplished through different strategies such as etching, growth, aggregation, and anti-aggregation. Based on these strategies, various sensors have been developed. This review focuses on the newly developed anti-aggregation-based strategy of plasmonic nanoparticles. Sensors based on this strategy have attracted increasing interest because of their exciting properties of high sensitivity, selectivity, and applicability. This review highlights LSPR-based anti-aggregation sensors, their classification, and role of plasmonic nanoparticles in these sensors for the detection of water pollutants. The anti-aggregation based sensing of major water pollutants such as heavy metal ions, anions, and small organic molecules has been summarized herein. This review also provides some personal insights into current challenges associated with anti-aggregation strategy of LSPR-based colorimetric sensors and proposes future research directions.

A turn-on unlabeled colorimetric biosensor based on aptamer-AuNPs conjugates for amyloid-β oligomer detection

Amyloid-β oligomers (AβO) have been identified as core biomarkers for early diagnosis of Alzheimer's disease (AD). For the first time, a "turn-on" unlabeled colorimetric aptasensor based on aptamer-polythymine (polyT)-polyadenine (polyA)-gold nanoparticles (pA-pT-apt@AuNPs) was developed for highly sensitive and specific detection of amyloid-β_1-40 oligomers (Aβ40-O). In this system, polyA sequence could preferentially anchor onto AuNPs surface as well as reduce the non-specific adsorption, and the aptamer could form upright conformation for the specific recognition of Aβ40-O. The aggregation of pA-pT-apt@AuNPs was induced by MgCl₂. However, the addition of Aβ40-O enabled the aptamer fold adaptively upon recognition and aptamer-Aβ40-O complex formed surrounding AuNPs, effectively stabilizing pA-pT-apt@AuNPs against salt-induced aggregation, therefore the color of pA-pT-apt@AuNPs solution still retained red. Based on this principle, the proposed aptasensor exhibited high sensitivity with the limit of detection of 3.03 nM and a linear detectable range from 10.00 nM to 100.0 nM. The superb sensitivity was achieved via the optimization of the length of polyA and polyT spacer. This pA-pT-apt@AuNPs based colorimetric aptasensor provides a rapid, cost-effective, highly sensitive detection method for Aβ40-O, which is valuable for the early diagnosis of AD.

Smartphone-Integrated Photoacoustic Analytical Device for Point-of-Care Testing of Food Contaminant Azodicarbonamide

Azodicarbonamide (ADA) is widely used as a flour additive due to its oxidizing and bleaching properties, but it reacts with wet flour during heat processing and is easily decomposed into semicarbazide with genotoxicity and carcinogenicity. In order to improve the efficiency of food safety supervision and expand the scope of food safety control, it is of great significance to develop a facile method for point-of-care testing (POCT) of ADA. Herein, a field-portable and universal smartphone-based photoacoustic (PA) integration device is constructed for quantitative POCT of ADA in flour. The recognition probe Prussian blue with favorable stability is loaded on a flexible substrate for fabricating a portable test strip. In the presence of target ADA, the PA signal changes driven by a modulated 808 nm laser beam can be conveniently collected through the recording application (Audio Lab) of the smartphone. By combining the economic test strip and portable PA device with smartphone readout, it not only greatly simplifies the operation steps but also dramatically reduces the size and cost of the instrument. There is a favorable linear relationship between the PA signal and ADA concentration in the range of 10-200 μmol L^-1 (R² = 0.9928), and a detection limit of 5 μmol L^-1 obtained is much lower than the maximum allowable ADA level in the extract of flour (388 μmol L^-1). The present miniature PA device with strong POCT ability holds enormous public health significance and economic value in the field of food safety, especially in resource-limited settings.

A handheld multifunctional smartphone platform integrated with 3D printing portable device: On-site evaluation for glutathione and azodicarbonamide with machine learning

Azodicarbonamide (ADA) in flour can be easily decomposed to semi-carbazide and biuret, exhibiting strong genotoxicity in vitro and carcinogenicity. Glutathione (GSH) can be conjugated with some ketone-containing compounds and unsaturated aldehydes to form toxic metabolites. Here, a novel ratio fluorescence probe based on blue emitting biomass-derived carbon dots (BCDs) and yellow emitting 2,3-diaminophenazine (OxOPD) was prepared for the bifunctional determination of glutathione (GSH) and ADA. This strategy includes three processes: (1) Ag⁺ oxidizes o-phenylenediamine (OPD) to produce OxOPD. The peak at 562 nm was enhanced, and the peak at 442 nm was reduced due to fluorescence resonance energy transfer (FRET), (2) glutathione binds Ag⁺ and inhibits the production of OxOPD, (3) ADA oxidizes GSH to form GSSG, resulting in the release of Ag⁺ by GSH. Therefore, the newly designed ratio fluorescence probe can be based on the intensity ratio (I₄₄₂/I₅₆₂) changes and significant fluorescent color changes to detect GSH and ADA. Moreover, a smartphone WeChat applet and a yolov3-assisted deep learning classification model have been developed to quickly detect GSH and ADA on-site based on an image processing algorithm. These results indicate that smartphone ratiometric fluorescence sensing combined with machine learning has broad prospects for biomedical analysis.

Radiation-Responsive Amino Acid Nanosensor Gel (RANG) for Radiotherapy Monitoring and Trauma Care

Accurate detection of doses is critical for the development of effective countermeasures and patient stratification strategies in cases of accidental exposure to ionizing radiation. Existing detection devices are limited by high fabrication costs, long processing times, need for sophisticated detection systems, and/or loss of readout signal over time, particularly in complex environments. Here, we describe fundamental studies on amino acid-facilitated templating of gold nanoparticles following exposure to ionizing radiation as a new colorimetric approach for radiation detection. Tryptophan demonstrated spontaneous nanoparticle formation, and parallel screening of a library of amino acids and related compounds led to the identification of lead candidates, including phenylalanine, which demonstrated an increase in absorbance at wavelengths typical of gold nanoparticles in the presence of ionizing radiation (X-rays). Evaluation of screening, i.e., absorbance data, in concert with chemical informatics modeling led to the elucidation of physicochemical properties, particularly polarizable regions and partial charges, that governed nanoparticle formation propensities upon exposure of amino acids to ionizing radiation. NMR spectroscopy revealed key roles of amino and carboxy moieties in determining the nanoparticle formation propensity of phenylalanine, a lead amino acid from the screen. These findings were employed for fabricating radiation-responsive amino acid nanosensor gels (RANGs) based on phenylalanine and tryptophan, and efficacy of RANGs was demonstrated for predicting clinical doses of ionizing radiation in anthropomorphic thorax phantoms and in live canine patients undergoing radiotherapy. The use of biocompatible templating ligands (amino acids), rapid response, simplicity of fabrication, efficacy, ease of operation and detection, and long-lasting readout indicate several advantages of the RANG over existing detection systems for monitoring radiation in clinical radiotherapy, radiological emergencies, and trauma care.

History, mechanism of action, and toxicity: a review of commonly used dough rheology improvers

Dough rheology improvers, which often are oxidative reagents in nature, have long been used in bread-making industry to enhance protein crosslinking and subsequently improve the dough rheological properties and bread qualities. Numerous studies were conducted to explore the effects of these oxidative agents on dough quality improving, however, the underlying mechanism of their action during dough development has not been fully understood. Due to the public health concerns, multiple oxidative reagents were banned in some countries across the world, while others are still permitted in accordance with regulations. Therefore, a comprehensive understanding of their application, significance, and safety in bread manufacturing is necessary. This review aims to provide a detailed information about the evolutionary history of several commonly used oxidants acting as dough rheology improvers, their mechanisms of action, as well as their potential toxicity.

A colorimetric sensing platform for azodicarbonamide detection in flour based on MnO2 nanosheets oxidative system

Azodicarbonamide (ADA), as a dough conditioner food additive in flour, can be turned into toxic biurea and semicarbazide after high temperature processing. Hence, the using of ADA in food material should be strictly controlled, and the detection of ADA is very important for consumers' safety and health. Herein, a simple and fast colorimetric strategy has been developed for ADA detection based on the MnO₂ nanosheets-3,3',5,5'-tetramethylbenzidine (TMB)-glutathione (GSH) as oxidative sensing system (MnO₂-TMB-GSH). Since the ADA can selectively react with GSH via oxidizing the sulfydryl (-SH) group of GSH to disulfide bond (S-S), which makes GSH unable to reduce MnO₂ nanosheets and restore its oxidase-like activity. The absorbance changes of the TMB solution depended on ADA content. The MnO₂-TMB-GSH colorimetric platform can detect the ADA with a linear range of 10 μmol L^-1 (11.6 ppm) to 400 μmol L^-1 (464 ppm), and the limit of detection (LOD) is 3.3 μmol L^-1 (3.51 ppm). Some potential interferences in real sample were tested and did not affect the MnO₂-TMB-GSH colorimetric platform for ADA detection. Furthermore, the sensing platform was applied for detecting ADA in real flour sample with a recovery of 96%-105% (RSD < 5%). This colorimetric method can effectively and rapidly detect ADA additives in flour less than the prescribed standard (45 mg kg^-1), which shows a great potential for visualization analysis and on-site detection of ADA in flour. A simple and fast colorimetric strategy has been developed for azodicarbonamide (ADA) detection based on the MnO₂ nanosheets-3,3',5,5'-tetramethylbenzidine (TMB)-glutathione (GSH) as oxidative sensing system (MnO₂-TMB-GSH).

Highly uniform self-assembled monolayers of silver nanospheres for the sensitive and quantitative detection of glutathione by SERS

The homeostasis and imbalance of glutathione (GSH), an important antioxidant in organisms, are one of the key signals that reflect the health of organisms. In this paper, a novel SERS sensing platform based on Ag film@Si that self-assembled using silver nanospheres was proposed, which was used for the highly sensitive and selective detection of GSH. With the aid of an oil/water/oil three-phase system, the nano-silver film was self-assembled and finally deposited on silicon wafers. The heterobifunctional crosslinking agent N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP), which contains pyridine rings and disulfide bonds, was involved in the exchange reaction between the sulfhydryl groups and disulfide bonds. With the addition of GSH, the breakage of disulfide bonds was promoted, thereby enhancing the SERS signal of SPDP. GSH can be detected sensitively by detecting the changes in the SPDP signal. The detection limit of GSH is 10 nM, and the method is still highly stable when the external environment is serum or other more complex environments.

Rapid and selective detection of aluminum ion using 1,2,3-triazole-4,5-dicarboxylic acid-functionalized gold nanoparticle-based colorimetric sensor

A highly selective, sensitive, rapid, low-cost, simple and visual colorimetric system for Al³⁺ ion detection was developed based on gold nanoparticles (AuNPs) modified with 1,2,3-triazole-4,5-dicarboxylic acid (TADA). The modified gold nanoparticles (TADA–AuNPs) were first prepared by sodium citrate (Na₃Ct) reduction of chloroauric acid (HAuCl₄) and then capped with a TADA ligand. Five TADA–AuNPs sensors were constructed with sodium citrate (Na₃Ct)/chloroauric acid (HAuCl₄) under different molar ratios. Results showed that the molar ratio of Na₃Ct/HAuCl₄, TADA–AuNPs concentration, pH range and detection time had obvious influences on the performance of this colorimetric method. The optimal detection conditions for Al³⁺ ions were as follows: Na₃Ct/HAuCl4 molar ratio of 6.4 : 1, 0.1 mM of TADA–AuNPs concentration, 4–10 pH range and 90 s of detection time. Under the optimal conditions and using diphenyl carbazone (DPC) as a Cr³⁺ masking agent, this colorimetric sensor exhibited outstanding time efficiency, selectivity and sensitivity for Al³⁺ detection. In particular, the detection limits of this sensor obtained via UV-vis and the naked eye were 15 nM and 1.5 μM, respectively, which were much lower than the current limit (3.7 μM) for drinking water in WHO regulation and better than the previous reports. Moreover, this colorimetric sensing system could be used to for on-site, trace level and real-time rapid detection of Al³⁺ in real water samples.

A colorimetric sensor based on TADA–AuNPs accompanied by a masking agent DPC was constructed, with which the rapid quantification of Al³⁺ can be realized by UV-vis spectroscopy or naked eye observation.

Nanoplasmonic sensor array for the detection and discrimination of pesticide residues in citrus fruits

Great attention has been directed towards developing rapid and straightforward methods for the identification of various pesticides that are usually used simultaneously in citrus fruits. The extensive use of diverse classes of pesticides in citrus fruits and their high toxicity may cause serious diseases in the human body. In the current study, a non-enzymatic sensor array has been developed for the identification and discrimination of five different pesticides belonging to diverse classes, including organophosphate, carbamate, and bipyridylium. For this aim, two gold nanoparticles (AuNPs) with different capping agents, citrate and borohydride, were used as sensing elements. The aggregation-induced spectra alterations of AuNPs were utilized to identify the pesticides in a wide range of concentrations (20-5000 ng mL-1). We have employed data visualization methods (i.e., heat maps, bar plots, and color difference maps), a supervised pattern recognition method (i.e., linear discrimination analysis), and partial least squares regression to qualitatively and quantitatively determine the pesticides. Finally, the practical applicability of the developed sensor array was evaluated for the identification of target pesticides in lime peel. The outcomes revealed that the probe could accurately verify the absence or presence of the pesticides in lime fruit.

Visual determination of azodicarbonamide in flour by label-free silver nanoparticle colorimetry

A very practical and competitive sensing strategy for the detection of azodicarbonamide in flour samples was developed by using label-free Ag NPs as a colorimetric probe. Well-dispersed Ag NPs in suspension can form aggregates upon reacting with glutathione (GSH) via Ag-SH covalent bonds and electrostatic attraction, with the color changing from bright yellow to red. However, azodicarbonamide can oxidize the -SH of GSH, preventing the aggregation of Ag NPs. Under the optimum conditions, the A₅₅₀/A₃₉₈ of Ag NPs is linearly related to the concentration of azodicarbonamide in the range of 0.33 μM to 1.7 μM. The proposed method can be used for the detection of azodicarbonamide in flour, with a detection limit of 0.09 μM and recovery between 95% and 97.4% (RSD < 6%). When the azodicarbonamide concentration reaches 0.33 μM, the color change can be detected by the naked eye.

Dynamically Tunable Plasmonic Band for Reversible Colorimetric Sensors and Surface-Enhanced Raman Scattering Effect with Good Sensitivity and Stability

A colorimetric sensor based on plasmonic nanoparticles (NPs) is a promising and convenient detection tool, but its reproducibility and adjustability remain a challenge because the NPs are mainly random and uncontrollable. Herein, a colorimetric sensor with good reversibility and reproducibility was prepared by embedding the two-dimensional (2D) Au NP arrays on the surface of the polyacrylamide hydrogel film to form 2D Au NP arrays attached a hydrogel composite. For this composite, with the change of the interspacing distance of Au NPs driven by the swelling-shrinking behavior of the hydrogel carrier, the diffraction peaks faded away and plasmonic coupling peaks appeared, accompanied by a series of obvious color changes (iridescence ↔ violet ↔ golden yellow ↔ red), which can be correlated to the applied water content. Importantly, the composite had good reproducibility as a result of a highly ordered array structure. Additionally, this colorimetric sensor with a dynamically tunable plasmonic band can be used as a high-quality surface-enhanced Raman scattering (SERS) substrate because the gap distance of the Au NPs can be uniformly controlled. We demonstrated that, as the active gap distance decreased, the SERS signals can be significantly intensified. When the water content reached 40%, this SERS substrate exhibited high sensitivity (10^-10 M for 4-aminothiophenol and 10^-9 M for thiram) and good reproducibility (relative standard deviation of <20%) using the excitation laser of 785 nm because of the small gap between two adjacent Au NPs and the highly ordered periodic structure. Such 2D Au NP arrays attached to a hydrogel composite could be a new strategy to obtain a high-quality colorimetric sensor and dynamic SERS substrate.

Noble Metal Nanoparticles Applications: Recent Trends in Food Control

Scientific research in the nanomaterials field is constantly evolving, making it possible to develop new materials and above all to find new applications. Therefore, nanoparticles (NPs) are suitable for different applications: nanomedicine, drug delivery, sensors, optoelectronics and food control. This review explores the recent trend in food control of using noble metallic nanoparticles as determination tools. Two major uses of NPs in food control have been found: the determination of contaminants and bioactive compounds. Applications were found for the determination of mycotoxins, pesticides, drug residues, allergens, probable carcinogenic compounds, bacteria, amino acids, gluten and antioxidants. The new developed methods are competitive for their use in food control, demonstrated by their validation and application to real samples.

Reproducible mesoporous silica-coated gold@silver nanoprobes for the bright colorimetric sensing of ascorbic acid

Herein, a colorimetric approach for the detection of ascorbic acid (AA) was developed by controlling the surface chemistry of silica-coated gold nanorod@silver nanoparticles (AuNR@Ag@mSiO₂).