Colorimetric Strategy Research Articles

Aptamer-based and sensitive label-free colorimetric sensing of phenylalanine via cascaded signal amplifications

The concentration variation of phenylalanine (Phe), an essential amino acid in humans, can cause metabolism disorders and even mental disability. Sensitive and convenient monitoring of Phe is therefore important for disease diagnosis. We describe here the establishment of a new aptamer-based, sensitive and label-free colorimetric Phe detection strategy by integrating catalytic hairpin assembly (CHA) and Mg2+-dependent DNAzyme amplification cascades. The target Phe coordinates with pentamethylcyclopentadienyl rhodium(III) chloride dimer [(Cp*RhCl2)2] to form a complex that has a high affinity to the corresponding aptamer sequence. Upon its binding to the aptamers in DNA duplex probes, ssDNA strands are released to trigger subsequent CHA reactions for the formation of many DNAzymes, which cleave the substrate signal probes to liberate lots of CHA initiation strands and free G-quadruplexes to realize the cascaded amplifications. Hemin further associates with the many G-quadruplexes to yield hemin/G-quadruplex mimicking peroxidases, which catalyze solution of substrate to exhibit highly enhanced UV–vis adsorption for detecting Phe at 0.19 μM level. At the meantime, the monitoring of Phe in diluted serums with high selectivity has also been demonstrated by the developed method, indicating its potential for simple diagnosis of Phe-related diseases.

Screening of Single-Stranded DNA Aptamer Specific for Florfenicol and Application in Detection of Food Safety.

In this work, the single-stranded DNA (ssDNA) aptamers specific to florfenicol (FF) and having a high binding affinity were prepared using the magnetic bead-based systematic evolution of ligands by the exponential enrichment technique (MB-SELEX). After 10 rounds of the MB-SELEX screening, aptamers that can simultaneously recognize FF and its metabolite florfenicol amine (FFA) were obtained. The aptamer with the lowest dissociation constant (Kd) was truncated and optimized based on a secondary structure analysis. The optimal aptamer selected was Apt-14t, with a length of 43 nt, and its dissociation constant was 4.66 ± 0.75 nM, which was about 7 times higher than that of the full-length sequence. The potential binding sites and interactions with FF were demonstrated by molecular docking simulations. In addition, a colorimetric strategy for nanogold aptamers was constructed. The linear detection range of this method was 0.00128–500 ng/mL and the actual detection limit was 0.00128 ng/mL. Using this strategy to detect florfenicol in actual milk and eggs samples, the spiked recoveries were 88.9–123.1% and 84.0–112.2%, respectively, and the relative standard deviation was less than 5.6%, showing high accuracy.

Colorimetric Approach for Nucleic Acid Salmonella spp. Detection: A Systematic Review.

Water- and food-related health issues have received a lot of attention recently because food-poisoning bacteria, in particular, are becoming serious threats to human health. Currently, techniques used to detect these bacteria are time-consuming and laborious. To overcome these challenges, the colorimetric strategy is attractive because it provides simple, rapid and accurate sensing for the detection of Salmonella spp. bacteria. The aim of this study is to review the progress regarding the colorimetric method of nucleic acid for Salmonella detection. A literature search was conducted using three databases (PubMed, Scopus and ScienceDirect). Of the 88 studies identified in our search, 15 were included for further analysis. Salmonella bacteria from different species, such as S. Typhimurium, S. Enteritidis, S. Typhi and S. Paratyphi A, were identified using the colorimetric method. The limit of detection (LoD) was evaluated in two types of concentrations, which were colony-forming unit (CFU) and CFU per mL. The majority of the studies used spiked samples (53%) rather than real samples (33%) to determine the LoDs. More research is needed to assess the sensitivity and specificity of colorimetric nucleic acid in bacterial detection, as well as its potential use in routine diagnosis.

Iron-doped cerium/nucleotide coordination polymer as highly efficient peroxidase mimic for colorimetric detection of fluoride ion.

A new coordination polymer (Ce-Fe-GMP) with excellent catalytic activity was preparedby a facile route, which was further applied to the detection of F- with high sensitivity and selectivity. The simple doping of Fe3+ into the coordination network can easily modulate the mixing ratio of Ce3+ and Ce4+ in the presence of H2O2, which can extremely improve the catalytic ability of Ce-Fe-GMP. Based on the synergistic effect, the Ce-Fe-GMP with dual-active sites shows better peroxidase activity than that of Ce-GMP. In addition, we foundthat F- can inhibit the peroxidase activity of Ce-Fe-GMP because of the coordination structure fragmentation and the regulation of Ce3+/Ce4+ ratio. Therefore, different concentrations of F- can be detected by the colorimetric reaction based on this mechanism. Theabsorption at 652nm displays a good linear relationship versus the concentration of F- over the range 2.0 to 100.0μM. Furthermore, F- in real mineral-mixed samples can be measured with satisfactory results. The colorimetric strategy based on the peroxidase activity of Ce-Fe-GMP is simple and low-cost, which shows the potential applications in the field of on-site environment measurement.

Light-triggered oxidative activity of chromate at neutral pH: A colorimetric system for accurate and on-site detection of Cr(VI) in natural water

Hexavalent chromium (Cr(VI)), a confirmed human respiratory carcinogen, has attracted widespread attention due to its high toxicity and mobility in natural water systems. However, the developed catalytic colorimetric probes for Cr(VI) detection remain challenging, such as various species of Cr(VI) coexisting in water or acid buffer, the obvious interference from oxidizing ions at acidic pH, the synthesis of complex materials, and the consumption of hydrogen peroxide. In this work, a phenomenon of UV light-triggered chromate to produce singlet oxygen under neutral pH was found. An environmentally-friendly method based on the rapid oxidation of chromogenic substrates was thus developed for the highly specific and accurate detection of Cr(VI). Cr(VI) serves as both the active center and the target of this system, which ensures the rapidity, simplicity, and environmental protection of the proposed colorimetric strategy. Importantly, the neutral catalytic condition ensures that the predominant species CrO42- can be accurately quantified and avoids the interference of oxidizing ions. Subsequently, a portable analytical system integrating an array of LED lamps, a miniature color reading device, and a developed smartphone-based program was successfully developed to accomplish high-throughput and on-site analysis of Cr(VI) in natural water. The proposed method provides a rapid screening and water quality assessment tool for the faster, more cost-effective, and accurate detection of Cr(VI) in the water system.

Colorimetric nano-beacon and magnetic separation-based rapid and visual assay for gram-negative bacteria

Food-borne diseases caused by pathogenic bacteria are one of the serious factors affecting human health. However, the most commonly used detection methods for pathogenic bacteria not only require expensive instruments, but also take a long time due to the complicated and cumbersome detection process. Therefore, the development of a fast, simple, and low-cost detection method for pathogenic bacteria is crucial for food safety and human health. In this work, based on the high binding ability of antimicrobial peptide (AMP) and polymyxin B (PMB) to bacteria, combined with magnetic separation technology, a new enzyme-free colorimetric strategy was constructed to achieve visual detection of Gram-negative bacteria in complex samples. The sensor system was divided into the following two parts: a colorimetric signal amplification nanoprobe, which was modified with AMP to enable effective binding of the colorimetric probe to the surface of bacteria, and a PMB-modified magnetic nanobead (MNB), which was used as the capture and enrichment unit of Gram-negative bacteria, as a result of which PMB could effectively distinguish Gram-negative bacteria from Gram-positive bacteria. Under optimized conditions, the detection limit of the method for Gram-negative bacteria (e.g. E. coli (G−)) was as low as 10 CFU/mL, and it was successfully applied to complex real samples. In addition, the developed colorimetric sensor offered advantages, such as fast response, less time consumption, high sensitivity, and low cost. It can be expected to become a new diagnostic tool for on-site detection of pathogenic bacteria in remote areas.

Cobalt-embedded nitrogen-doped carbon nanosheets with enhanced oxidase-like activity for detecting perfluorooctane sulfonate

In this study, we used leaf-like zeolitic imidazolate framework (ZIF-L) as the precursor, CoAc2⋅4H2O as the Co source, and 1,10-phenanthroline (Phen) as the N source to synthesize the cobalt-embedded nitrogen-carbon nanosheet (CoNCN) with enhanced enzyme-like activity. A variety of characterizations proved that the ultra-thin two-dimensional nanosheet structure of the CoNCN with both micropores and mesopores exposes more Co-Nx active sites, which makes it easier for the colorimetric substrate to contact the CoNCN’s surface. Compared with the Co-NC (the synthesized product without using Phen) and NC (the synthesized product without using CoAc2⋅4H2O) controls, the optimized CoNCN nanozyme exhibits 2.3-fold and 12.3-fold enhancement of oxidase-like activity, and 1.3-fold and 25.0-fold enhancement of catalase-like activity, respectively. In addition, the CoNCN has a high affinity for TMB, with a Km value of 0.53 mM and a Vmax value of 3.24 × 10-7 M s−1. The mechanism investigation indicated that the •OH, O2•− and 1O2 were reactive oxygen species responsible for the oxidase-like activity of CoNCN. The perfluorooctane sulfonate (PFOS) can fade the blue oxTMB. Based on this, an analytical method based on a colorimetric strategy coupled to classical univariate calibration was established for the sensitive detection of PFOS, with a linear range of 0.01 to 100 μM, and the limit of detection (LOD) of 20 nM according to modern IUPAC definition. The proposed method was successfully used to determine the concentration of PFOS in actual water samples.

Colorimetric detection of total antioxidants in green tea with oxidase-mimetic CoOOH nanorings

Green tea is a popular beverage and is widely consumed due to its taste and antioxidative polyphenols. Herein, a smartphone-based colorimetric reader using cobalt oxyhydroxide (CoOOH) nanorings has been successfully applied to detect antioxidants in green tea with high reliability and robustness. By exploiting the oxidase-mimicking activity, the as-synthesized CoOOH nanorings replaces natural enzymes to directly catalyze oxidate colorless 3,3 ´ ,5,5 ´ -tetramethylbenzidine (TMB), while antioxidants can disintegrate CoOOH, leading to an antioxidant concentration-dependent color change. Benefiting from the CoOOH nanorings-based colorimetric strategy, a smartphone-assistant nanosensor was devised for portable and visual detection of antioxidants in green tea. The proposed method can be extended to visual detection of a diverse range of diseases by responding to their specific antioxidant, and thus provide a pivotal disease toolbox that is compatible for development at the point-of-care.

Visible Light-Responsive Sulfone-Based Covalent Organic Framework as Metal-Free Nanoenzyme for Visual Colorimetric Determination of Uranium

Covalent organic framework (COF) has been attracting considerable attention as a novel crystalline material owing to its extended π-electron conjugation and excellent spectral behavior. In this study, we present an imine-linked two-dimensional (2D) crystalline sulfone-based covalent organic framework (TAS-COF) synthesized by 2,4,6-triformylphloroglucinol (Tp) and 3,7-diaminodibenzo[b,d]thiophene (DAS) via a Schiff base condensation reaction. The benzothiophene sulfone endows the as-synthesized TAS-COF with excellent oxidase-like activity under visible light irradiation, ascribed to the generation of superoxide radicals (O2•−) by photo-generated electron transfer. TAS-COF can efficiently oxidase the colorless substrate 3,3′,5,5′-tetramethylbenzydine (TMB) into blue oxidized TMB (oxTMB) when exposed to visible light, and the presence of uranium (UO22+) leads to clear color fading due to the coordination between the imine of oxTMB and UO22+. A colorimetric strategy is thus developed for UO22+ determination with a detection limit of 0.07 μmol L−1. Moreover, a paper-based visual sensing platform is also constructed to offer simple and fast UO22+ content evaluation in water samples. The present study not only provides a promising strategy to prepare visible light-triggered COF-based metal-free nanoenzymes but also extends the applications of COF material in radionuclide detection.

A portable dual-mode colorimetric platform for sensitive detection of Hg2+ based on NiSe2 with Hg2+-Activated oxidase-like activity

The nanozyme-based colorimetric strategy for heavy metal detection has broad application prospects nowadays. However, the inefficient recognition capabilities of nanozyme sensors for targets hinder its further application. Herein, the authors synthesize bare nickel selenide (NiSe2) via a one-step hydrothermal reaction, in which the Se element possesses a strong binding ability with mercury (Hg). As expected, NiSe2 exhibits oxidase-like activity in the presence of Hg2+, that is, Hg2+ can enhance the oxidase-like activity of NiSe2. The enhanced mechanism is the accelerated electron transfer between NiSe2–Hg2+ and substrate caused by the formation of Hg–Se bonds. Besides, the oxidase-like activity of NiSe2 exhibits excellent selectivity, sensitivity and stability in response to Hg2+, which enables NiSe2–Hg2+ to efficiently oxidize colorless TMB to blue TMB even in harsh environments. Based on this, a dual-mode colorimetric sensor integrating solution reaction and test paper is developed for the detection of Hg2+. In the Hg2+ concentration range of 10–700 nM, the colorimetric platform presents a liner response to Hg2+, which can reach a low LOD of 5.18 nM in solution reaction and 8.42 nM in the test paper. The proposed strategy can also be applied to real water samples with good recovery and excellent self-calibration capability.

One-pot fabrication of nanozyme with 2D/1D heterostructure by in-situ growing MoS2 nanosheets onto single-walled carbon nanotubes with enhanced catalysis for colorimetric detection of glutathione

A nanozyme with 2D/1D heterostructure has been fabricated by the in-situ growth of molybdenum disulfide nanosheets (MoS2 NSs) onto single-walled carbon nanotubes (SWCNTs). It was discovered that the so-obtained SWCNTs@MoS2 nanozyme could exhibit greatly improved peroxidase-like catalysis, due to that the formed 2D/1D interfacial coupling in the heterostructure might provide more active sites and exhibit enhanced charge transferring during the catalytic reactions, as confirmed by the X-ray photoelectron spectroscopy, photoluminescence, electrochemical impedance spectra and radical capturing experiments. Furthermore, the catalysis of the developed nanozyme could be selectively inhibited by glutathione (GSH) through the competitive consumption of hydroxyl radicals with enzyme substrate in the catalytic reaction system. A SWCNTs@MoS2 catalysis-based colorimetric strategy was further proposed for the quantitative analysis of GSH with the concentrations linearly ranging from 0.01 to 1000.0 μM. Besides, the feasibility of the developed colorimetric method was evaluated by monitoring GSH separately in the extractions from hela cells and human serum, promising the extensive applications for monitoring various biological species like GSH in the clinical laboratory. Importantly, such a fabrication route for nanozyme with 2D/1D heterostructure may pave the way towards the wide applications for designing various nanzymes with improved catalysis.

Colorimetric detection of Salmonella typhimurium based on hexadecyl trimethyl ammonium bromide-induced supramolecular assembly of β-cyclodextrin-capped gold nanoparticles.

We developed an effective and specific colorimetric strategy to detect Salmonella typhimurium (S. typhimurium) based on hexadecyl trimethyl ammonium bromide (CTAB)-induced supramolecular assembly of β-cyclodextrin-capped gold nanoparticles (β-CD-AuNPs). In this study, ssDNA aptamer of S. typhimurium could combine with CTAB to form the supramolecular ssDNA-CTAB composite, so the ssDNA aptamer was applied to control the concentration of CTAB. In the presence of S. typhimurium, ssDNA aptamers selectively bound to S. typhimurium but not to CTAB, leading to the host-guest chemistry reaction of CTAB and β-CD resulting in β-CD-AuNP supramolecular assembly aggregation with an obvious color change. The ratio of absorption at 650 and 520nm (A650nm/A520nm) has a linear correlation to the log scale of the concentration of the bacteria (1 × 102-1 × 107CFU/mL) with a low limit of detection (LOD) of 13CFU/mL. In addition, this optical sensor has good selectivity and practicability. In milk samples, the recovery was 93.55-111.32%, which suggested its potential application in real samples.

Polymer-assisted Au@PDA nanoparticles lyophilized powder with high stability and low adsorption and its application in colorimetric biosensing

Gold nanoparticles (Au NPs) has been widely used to develop label-free colorimetric biosensors. Since the lyophilization process of Au NPs might cause various stresses and lead to irreversible aggregation, Au NPs were usually preserved in an aqueous suspension, which was inconvenienced for transportation and storage. In addition, the potential adsorption interaction between target and Au NPs was often ignored, which may lead to false-signal for Au NPs based colorimetric strategy. Herein, polydopamine-coated gold nanoparticles (Au@PDA NPs) freeze-dried powder was prepared with the assistance of polyvinylpyrrolidone (PVP) (i.e. Au@PDA-PVP NPs) or polyethylene glycol (PEG) (i.e. Au@PDA-PEG NPs). After freeze-dried powder of Au@PDA nanoparticles was redissolved, not only their spectral properties can still be maintained, but also the Au@PDA nanoparticles have nice monodispersity. Besides, the freeze-dried powder has long-term stability and could be stored for at least nine months. Since kanamycin, an aminoglycoside antibiotic, can be absorbed on the surface of Au NPs and induce easily the false signal, it was difficult to be detected using conventional Au NPs-based colorimetric method. Thus, kanamycin was chosen as the model target, a simple, sensitive and label-free colorimetric sensor was established. Given that the adsorption between kanamycin and Au@PDA-PVP NPs was effectively avoided, the possibility of false-positive signal was also reduced. The detection limit of kanamycin was 0.22 nM (S/N = 3), which was met the requirements for the detection of kanamycin residues in milk. This work not only provided an effective and facile way to prepare the nanomaterial lyophilized powder, but also expanded the application of the Au NPs based colorimetric method.

A colorimetric assay for cholesterol based on the encapsulation of multienzyme in leaf-shape crossed ZIF-L

The serum cholesterol level is an important indicator of healthy and there is a great necessity for frequent cholesterol monitoring to some cardiovascular-related diseases, which puts forward higher requirements for point-of-care testing (POCT) of cholesterol. In this work, a cascade catalytic system of cholesterol is developed by encapsulation of cholesterol oxidase (ChOx) and PdCuAu nanoparticles into zeolitic imidazolate framework-L (ChOx/PCA@ZIF-L). Results indicate that ZIF-L carrier can significantly increase the catalytic activity of single or multiple enzymes, due to its high loading capacity and efficient molecular transport. Under the optimal conditions, the absorbance of reaction system performs linear relationships with the concentration of cholesterol in two intervals from 0.0005 mmol/L to 1.0000 mmol/L, with a limit of detection of 0.2176 µmol/L. The proposed colorimetric strategy based on ChOx/PCA@ZIF-L performs a good agreement with the results provided by chemiluminescence method for the serum cholesterol detection. Interestingly, a simple paper-based sensing system is constructed through a pre-reaction-transfer operation, which gets rid of the complex pre-processing requirements of traditional operations on filter paper. The presented strategy allows for the sensitive, convenient, costless assay of serum cholesterol, and paves a new way to design the POCT device for daily monitoring of healthy.

Enzyme-controllable just-in-time production system of copper hexacyanoferrate nanoparticles with oxidase-mimicking activity for highly sensitive colorimetric immunoassay

Nanozymes are a series of elaborately designed nanomaterials that can mimic the catalytic sites of natural enzymes for reactions. Bypassing the tedious design and preparation of nanomaterial, in this work, we report on a novel just-in-time production system of copper hexacyanoferrate nanoparticles (CHNPs), which act as an oxidase-mimicking nanozyme. This system can rapidly produce CHNPs nanozyme on demand by simply mixing Cu(II) with potassium hexacyanoferrate(III) (K3[Fe(CN)6]). It is found that once K3[Fe(CN)6] is reduced to K4[Fe(CN)6], the formation of CHNPs is inhibited. Therefore, the just-in-time production system of CHNPs was coupled with alkaline phosphatase (ALP) to construct an enzyme-controllable just-in-time production (ECJP) system, in which ALP could inhibit the production of by catalyzing the hydrolysis of ascorbic acid 2-phosphate (AAP) to generating ascorbic acid (AA). The ECJP system is then used to probe the activity of ALP by employing 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) as the chromogenic substrate, and a detection limit of 0.003 U L−1 was achieved. Moreover, by adapting ALP as the enzyme label, an ECJP system-based colorimetric immunoassay protocol was established for sensitive detection of aflatoxin B1 (AFB1), and a detection limit as low as 0.73 pg mL−1 was achieved. The developed immunoassay method is successfully applied to the detection of AFB1 in peanut samples. The operation of ECJP system is quite simple and the coupling of ALP with CHNPs nanozyme can arouse dual enzyme-like cascade signal amplification. So, we believe this work can offer a new perspective for the development of nanozymes-based biodetection methods and colorimetric immunoassay strategies.

Filter paper-based colorimetric analysis: An instrument-free strategy for semiquantitative naked-eye detection of food colorants

A filter paper-based colorimetric strategy for instrument-independent visual detection of artificial food colorants (FCs) was developed in this study. Indicator papers were prepared via the one-step polycondensation of silane coupling agents onto glass microfiber filter papers, and colorimetric cards with a fine one-to-one correspondence between their colors and FCs concentrations were straightforward obtained by the extraction of FCs with indicator papers by virtue of electrostatic interaction and hydrophobic effect. Filter papers post-modified via such a simple way were proved to be of improved binding class selectivity and colorimetric sensitivity, allowing for in-situ colorimetric assay of FCs in an unprecedently wide range of applicable pH (1.0–12.0) with high reliability and fine versatility. Finally, the semiquantitative naked-eye determination of FCs (Allura red, brilliant blue and sunset yellow) in real-world drink samples was experimentally confirmed to be feasible by comparison with the findings of UV–vis absorption spectra, HPLC and mass spectra.

SERS and Indicator Paper Sensing of Hydrogen Peroxide Using Au@Ag Nanorods.

The detection of hydrogen peroxide and the control of its concentration are important tasks in the biological and chemical sciences. In this paper, we developed a simple and quantitative method for the non-enzymatic detection of H2O2 based on the selective etching of Au@Ag nanorods with embedded Raman active molecules. The transfer of electrons between silver atoms and hydrogen peroxide enhances the oxidation reaction, and the Ag shell around the Au nanorod gradually dissolves. This leads to a change in the color of the nanoparticle colloid, a shift in LSPR, and a decrease in the SERS response from molecules embedded between the Au core and Ag shell. In our study, we compared the sensitivity of these readouts for nanoparticles with different Ag shell morphology. We found that triangle core–shell nanoparticles exhibited the highest sensitivity, with a detection limit of 10−4 M, and the SERS detection range of 1 × 10−4 to 2 × 10−2 M. In addition, a colorimetric strategy was applied to fabricate a simple indicator paper sensor for fast detection of hydrogen peroxide in liquids. In this case, the concentration of hydrogen peroxide was qualitatively determined by the change in the color of the nanoparticles deposited on the nitrocellulose membrane.

Endonuclease-driven DNA walking for constructing a novel colorimetric and electrochemical dual-mode biosensing method

The development of methods to realize the on-site analysis of antibiotic pollutants is of great importance for food quality control and environmental monitoring. Herein, we designed a magnetic bead (MB)-based DNA walker and utilized its target-triggered and endonuclease-driven walking reaction to develop a novel colorimetric and electrochemical dual-mode biosensing method for the convenient detection of kanamycin (Kana) antibiotic. The colorimetric signal transduction strategy of the method was constructed on the telomerase extension of the DNA walking-released telomeric primer into G-quadruplex/hemin DNAzymes. Due to the DNA walking and telomerase dual signal amplification, a good linear relationship from 0.1 pg mL−1 to 1 ng mL−1 was obtained for this strategy with a detection limit of 22 fg mL−1. Meanwhile, the MB complex produced through the above DNA walking reaction was also used as a multipedal DNA walker to develop an electrochemical signal transduction strategy. By utilizing it to trigger another endonuclease-driven DNA walking at a DNA hairpin-modified electrode, ferrocene labels were quantitatively released from this electrode to cause the electrochemical signal decrease. Because of the dual endonuclease-driven DNA walking for signal amplification, a five-order of magnitude wide linear relationship from 0.01 pg mL−1 to 1 ng mL−1 was obtained with an ultralow detection limit of 8.4 fg mL−1. As the two strategies did not involve complicated manipulations and the requirement of expensive instruments, this biosensing method exhibits a high application value for the on-site semiquantitative screening and accurate analysis of antibiotic residues.

Highly sensitive visual colorimetric sensor for xanthine oxidase detection by using MnO2-nanosheet-modified gold nanoparticles

In this study, a highly sensitive colorimetric assay has been constructed for the determination of xanthine oxidase (XOD) activity by the GNP@MnO2 core-shell nanoparticles as probe. In the presence of XOD, xanthine can be oxidized to produce H2O2, which makes the MnO2 shell fallen off. With the single particle detection (SPD) based dark field microscopy (DFM), the scattering color of GNP@MnO2 NP probe shows obvious change before and after etching process. At the single particle level, noticeable color change of the single probe can be easily detected in the existence of trace XOD. This SPD-based colorimetric strategy displays broad linear dynamic range (0.02–4 mU/mL) and low detection limit of 7.82 μU/mL, which is more sensitive than the results from ensemble sample measurement. In addition, we tested the inhibitory effect of quercetin on the activity of XOD and obtained good inhibition effect. As a consequence, this SPD-based colorimetric strategy provides new perception for the ultrasensitive detection of molecules in complex system.

Portable silver-doped prussian blue nanoparticle hydrogels for colorimetric and photothermal monitoring of shrimp and fish freshness

Herein, we explored a colorimetric and photothermal readout strategy based on silver-doped prussian blue nanoparticles (SPB NPs) hydrogel for detection of trimethylamine (TMA) and monitoring of shrimp/fish freshness. The functional hydrogel is fabricated via the addition of SPB NPs to agarose hydrogel. TMA vapor can permeate into the functional hydrogel, thereby causing the decomposition of SPB NPs along with the color evolution (blue to colorless) and the disappearance of photothermal effects. The color information was digitized by a smartphone combined with RGB (red/green/blue) analysis. The photothermal effects was recorded by a handheld thermal imager. The ∆R gray value and temperature evolutions of the functional hydrogel were all closely related to the concentration of TMA, making it successful in the assessment of shrimp/fish freshness. Both colorimetric and photothermal signals are linear response to TMA concentration from 0.21 to 0.54 ppm. The application of smartphone and handheld thermal imager greatly improves the portability and practicality of monitoring on-spot. More significantly, the photothermal signal can still be applied to assess food freshness, when the hydrogel is contaminated by the colored food substrates and the colorimetric signal cannot be used to assessment. To our knowledge, this is the first time that a photothermal strategy has been developed to monitor food freshness. Therefore, this simple, cost-effective, nondestructive and sensitive dual-mode functional hydrogel will open up a new prospect for monitoring of food spoilage.