Target-induced Trivalent G-quadruplex/hemin DNAzyme for Colorimetric Detection of Hg^(2+) Based on an Exonuclease III Assisted Catalytic Hairpin Assembly

Mercury ion(Hg^(2+)),a highly noxious of heavy metalion,has detrimental effects on the ecological environment and human health.Herein,we have developed an exonuclease III(Exo III)assisted catalytic hairpin assembly formation of a trivalent G-quadruplex/hemin DNAzyme for colorimetric detection of Hg^(2+).A hairpin DNA(Hr)was designed with thymine-Hg^(2+)-thymine pairs that catalyzed by Exo III is prompted to happen upon binding Hg^(2+).A released DNA fragment triggers the catalytic assembly of other three hairpins(H1,H2,and H3)to form many trivalent G-quadruplex/hemin DNA enzymes for signal output.The developed sensor shows a dynamic range from 2 pM to 2μM,with an impressively low detection limit of 0.32 pM for Hg^(2+)detection.Such a sensor also has good selectivity toward Hg^(2+)detection in the presence of other common metal ions.This strategy shows the great potential for visual detection with portable type.

Selectively colorimetric detection of cysteine with triangular silver nanoprisms

Triangular silver nanoprisms were prepared and applied to make colorimetric detection of cysteine based on our findings that cysteine could lead to the blue shift of the dipole plasmon resonance absorption, but other 19 kinds of natural amino acids could not. Cysteine with a concentration 160 nmol/L can result in a color change that can be discerned with naked eyes.

Copper-based nanomaterials as peroxidase candidates for intelligent colorimetric detection and antibacterial applications

With natural polyphenol compound tannic acid(TA)as the ingredient,copper tannate(CuTA)nanolayer material was formed by self-assembly under the conditions of heating and copper ion.Copper ion and tannic acid could coordinate to form Cu-O-C structure which was similar to the Fe-N-C structure of natural heme enzymes.In addition,the obtained CuTA nanomaterial possessed excellent peroxidase-like catalytic activity and stability.Based on this excellent enzymatic activity,CuTA nanomaterials could be used for colorimetric detection of hydrogen peroxide with a smartphone and decomposition of organic dyes.Based on the structural characteristics and catalytic activity,the copper tannate nanozymes could also serve as potential antibacterial reagents to effectively inhibit the growth of gram-negative and positive bacteria in a low hydrogen peroxide level.This work may promote the exploration of novel peroxidase mimetics and broaden the applications of nanozymes.

Utilization of unmodified gold nanoparticles in colorimetric detection

This review begins with an overview of the appealing properties and various applications of gold nanoparticles, and briefly summarizes recent advances in using unmodified gold nanoparticles to detect different kinds of targets including nucleic acids, proteins, metal ions and small organic molecules. The key point to the unmodified gold nanoparticle-based visual detection assay is to control dispersion and aggregation of colloidal nanoparticles by targets of interest, which usually relies on affinities between gold nanoparticles and targets. The degree of dispersion or aggregation can be visualized through the change of the solution color or the precipitation of nanoparticles from the solution. Thus, the existence of the target molecules can be trans-lated into optical signals and monitored by the naked eye conveniently. Finally, some future prospects of this research field are given.

Enhanced peroxidase-like activity of hierarchical MoS2-decorated N-doped carbon nanotubes with synergetic effect for colorimetric detection of H2O2 and ascorbic acid

The exploitation of multifunctional nanocomposites is highly desired in environmental monitoring,biosensors,and medical diagnosis.In this paper,a simple approach has been proposed to fabricate MoS2 decorated N-doped carbon nanotubes(NCNTs@MoS2)hybrid composites as efficient peroxidase-like mimics.The combination of the MoS2 and N-doped carbon nanotubes(NCNTs)brings about an enhanced synergistic effect,leading to remarkably decent intrinsic peroxidase-mimic activities than that of the single components.Due to the high catalytic efficiency of the resultant NCNTs@MoS2 hybrid nanotubes as peroxidase-like mimics,a co nvenient colorimetric approach for the sensitive determination of H2 O2 and ascorbic acid have been developed with a detection limit of about 0.14μmol/L and 0.12μmol/L,respectively.The work offers a new strategy for the fabrication of peroxidase-like nanomaterials with excellent catalytic activity,which indicates great promising applications in sensitive detections in real samples.

Enhanced adsorption and colorimetric detection of tetracycline antibiotics by using functional phosphate/carbonate composite with nanoporous network coverage

This work presents efficient tetracycline(TC) antibiotics adsorption using a functional porous phosphate/carbonate composite(PCC). The PCC was fabricated by anion-exchange of phosphate on the surface of vaterite-phase calcium carbonate particle scaffolds. The PCC,having dense nanoporous network coverage with large surface area and pore volume, exhibited excellent TC adsorption in solution. Its adsorption isotherm fitted well to the Freundlich model, with a maximum adsorption capacity of 118.72 mg/g. The adsorption process was spontaneous, endothermic, and followed pseudo-second-order kinetics. From the XPS analysis, the hydrogen bonding and surface complexation were the key interactions in the process. In addition, a colorimetric TC detection method was developed considering its complexation with phosphate ions, originating from PCC dissolution, during adsorption.The method was used to detect TC in mg/L concentrations in water samples. Thus, the multifunctional PCC exhibited potential for use in TC removal and environmental remediation.

Biomineralization-inspired copper-cystine nanoleaves capable of laccase-like catalysis for the colorimetric detection of epinephrine

Recently,many efforts have been dedicated to creating enzyme-mimicking catalysts to replace natural enzymes in practical fields.Inspired by the pathological biomineralization behaviour of L-cystine,in this study,we constructed a laccase-like catalyst through the co-assembly of L-cystine with Cu ions.Structural analysis revealed that the formed catalytic Cu-cystine nanoleaves(Cu-Cys NLs)possess a Cu(I)-Cu(II)electron transfer system similar to that in natural laccase.Reaction kinetic studies demonstrated that the catalyst follows the typical Michaelis-Menten model.Compared with natural laccase,the Cu-Cys NLs exhibit superior stability during long-term incubation under extreme pH,high-temperature or high-salt conditions.Remarkably,the Cu-Cys NLs could be easily recovered and still maintained 76%of their activity after 8 cycles.Finally,this laccase mimic was employed to develop a colorimetric method for epinephrine detection,which achieved a wider linear range(9–455μmol·L^(−1))and lower limit of detection(2.7μmol·L^(−1)).The Cu-Cys NLs also displayed excellent specificity and sensitivity towards epinephrine in a test based on urine samples.

Colorimetric Detection of Glucose Using WO_(3) Nanosheets as Peroxidase-mimetic Enzyme

In the present work,WO_(3) nanosheets(WO_(3) NSs)were prepared by a facile method at room temperature.The obtained WO_(3) NSs showed peroxidase-like activity,which could catalyze 3,3’,5,5’-tetramethylbenzidine(TMB)to form a blue oxidation product(ox TMB)in the presence of H_(2)O_(2).Based on this,convenient and sensitive colorimetric methods for the detection of H_(2)O_(2) and glucose were established.The linear ranges for detecting H_(2)O_(2) and glucose were 1–200µmol/L and 1–100µmol/L,respectively.The limits of the detection of H_(2)O_(2) and glucose were as low as 0.79 and 0.96µmol/L,respectively.This method was also successfully applied to the detection of glucose in urine samples.The detection result was consistent with that of the value detected by the clinical method,indicating the potential in clinical diagnosis and biomedical detection.

Robust Colorimetric Detection of Cu^(2+) by Excessed Nucleotide Coordinated Nanozymes

Many sensors for Cu^(2+)rely on the catalytic activity of Cu^(2+)to produce a color change,but these colorimetric sensors often suffer from poor selectivity and are susceptible to interference.In this work,we found that Cu^(2+)-catalyzed TMB oxidation can be significantly improved with excessed nucleotides and nucleosides.Especially,with oversaturated guanosine 5′-monophos-phate(GMP)to form a coordination nanozyme,the catalytic efficiency was greatly accelerated.It can be attributed to the specific binding between the electron-rich oxygen and nitrogen atoms of guanine with Cu^(2+).A sensitive and selective strategy for Cu^(2+)sensing was proposed.Owing to the presence of excessed GMP,it ensures robust activity and less susceptibility to interference.This allowed us to test the sensor in complex samples such as the seawater.This work strongly suggests that by supplying excessed ligands,far exceeding the binding stoichiometry,we may produce more robust sensing systems.

Bimetallic AuRu aerogel with enzyme-like activity for colorimetric detection of Fe^(2+) and glucose

Aerogels have become a hot topic of research due to their extremely low density and special interconnected structure as well as their enzyme-like activity.The development of new multifunctional nano-enzyme aerogels with high activity and good stability is still a considerable challenge.In this paper,AuRu aerogels with peroxidase and oxidase activities were synthesized using a simple one-step method and successfully used to construct colorimetric sensors for the detection of Fe^(2+)and glucose based on their enzyme-like activities.Furthermore,we are fortunate to find that AuRu aerogels have good photothermal properties.This suggests that AuRu aerogels can be used not only for in vitro testing but also for promising applications such as disease treatment.

Colorimetric detection of D-amino acids based on anti-aggregation of gold nanoparticles

A new method has been proposed to realize the visual detection of D-amino acids （DAAs） via the antiaggregation of 4-mercaptobenzoic acid modified gold nanoparticles （AuNPs） in the presence of D-amino acid oxidase （DAAO）. The negatively charged AuNPs were prepared using sodium citrate as a reducer and stabilizer. The presence of 4-mercaptobenzoic acid （4-MBA） and Cu2＋ induces the aggregation of AuNPs, resulting in a color change from ruby red to royal purple. However, DAAO could oxidize DAAs to generate H2O2. In the presence of H2O2, the mercapto （-SH） group in 4-mercaptobenzoic acid can be oxidized to form a disulfide （-S-S-） bond. Based on these facts, the pre-incubation of DAAs and 4-mercaptobenzoic acid with DAAO would significantly reduce the concentration of free 4-mercaptobenzoic acid molecules, thus the aggregation of AuNPs was interrupted since due to the lack of inducer. As the concentration of DAAs increases, the color of the AuNPs solution would progress from royal purple to ruby red. Consequently, DAAs could be monitored by the colorimetric response of AuNPs using a UV-vis spectrophotometer or even naked eyes. This DAAO mediated visual detection method could determine D- alanine （D-Ala） as a representative DAA with concentrations ranging from 1.5 × 10^-7mol L 1 to 3.0 × 10^-5 mol L^-1, and the detection limit was as low as 7.5 × 10^-8 mol L^-1. The proposed method is convenient, low-cost and free of complex equipment, making it feasible to analyze the concentration of D-AIa in real samples of β-amyloid peptide （Aβ1-42）.

Bis-naphthalimide-based supramolecular self-assembly system for selective and colorimetric detection of oxalyl chloride and phosgene in solution and gas phase

Two bis-naphthalimide-based supramolecular gelators(NN-3 and NN-4)with a little difference of position of amino groups were designed and synthesized for the detection of oxaloyl chloride and phosgene.Energy transfer could be occurred between two naphthalimide groups in molecules NN-3 and NN-4.Yellow gels NN-3 and NN-4 were formed in some mixed solvents,and nanofibers with different size were obtained in these gels.The self-assembly processes of NN-3 and NN-4 in different solvents were investigated by UV-vis absorption,fluorescent spectra,SEM,FTIR,XRD and NMR.Gelators NN-3 and NN-4 could selectively detect oxaloyl chloride in solution and film states,but detect phosgene only in solution.NN-3exhibited the ratiometric detection ability towards oxaloyl chloride and phosgene with the low limit of detection(LOD)of 210 nmol/L and 90 nmol/L,respectively.NN-4 as the corresponding control sample,it owned the higher LOD towards oxaloyl chloride and phosgene of 12.4μmol/L and 64μmol/L,respectively.Interestingly,films NN-3 and NN-4 could sensitively detect oxaloyl chloride gases with the low LOD of2.0 ppm and 8.34 ppm,respectively.The detection mechanisms of NN-3 and NN-4 were well studied by1H NMR titration,HRMS and theoretical calculation.

Target-responsive DNAzyme hydrogel for portable colorimetric detection of lanthanide(Ⅲ)ions

Lanthanide elements(Ln)play an important role in industry and agriculture.As a result of the increasing consumption of lanthanides,environmental emission of Ln has become detrimental to the health of flora and fauna.Current methods for trace lanthanides detection mainly rely on sophisticated instruments.In this article,a Ln^(3+)dependent DNAzyme was incorporated into a hydrogel to generate Ln^(3+)sensitive DNAzyme hydrogel for portable colorimetric detection.The enzyme strand and its substrate strand act as crosslinker and functional unit of the hydrogel with polyacrylamide chains as the scaffold and gold nanoparticles(AuNPs)as the indicator of hydrogel stability.Any ions in the Ln^(3+)series can trigger the cleavage of substrate strand by activating the enzyme strand,thereby decreasing the crosslink ratio and leading to collapse of the hydrogel.The release of the encapsulated AuNPs turns the supernatant wine red.Using this colorimetric method,Ln^(3+)can be detected with high sensitivity,with a limit of detection(LOD)of 20 nM for Ce^(3+).The hydrogel responds specifically to any Ln^(3+)ion and works well with the spiked lake sample without the need of instruments and skilled operators.Our results suggest that the lanthanide responsive hydrogel can be used for portable and sensitive detection of Ln^(3+)contamination in the field.

Colorimetric detection of glucose using a boronic acid derivative receptor attached to unmodified AuNPs

A simple, cheap and non-enzymatic colorimetric strategy for glucose detection has been designed based on the interactions between a phenylboronic acid （PBA） derivative, which is coupled with gold nanoparticles （AuNPs） as the colorimetric reporters, and glucose. The PBA-AuNPs hybrid system proposed here exhibits ordered photochemistry behaviors upon the addition of glucose at different pH values. There are two linear regions of glucose concentration for the glucose sensor at different pH values, i.e., between 0.1 mmol/L and 9.8 mmol/L at pH 6 with the detection limit of 64μmol/L and between 0 and 6.5 mmol/L with the detection limit of 48 μmol/L at pH 9, respectively. To test the practicality of the sensor system, we also applied this assay to detect a glucose sample in the artificial saliva.

Reversible regulation of enzyme-like activity of molybdenum disulfide quantum dots for colorimetric pharmaceutical analysis

Regulating the catalytic activity of nanozymes is significant for their applications in various fields.Here,we demonstrate a new strategy to achieve reversible regulation of the nanozyme’s activity for sensing purpose.This strategy involves the use of zero-dimensional MoS;quantum dots(MQDs)as the building blocks of nanozymes which display very weak peroxidase(POD)-like activity.Interestingly,such POD-like activity of the MQDs largely enhances in the presence of Fe;while diminishes with the addition of captopril thereafter.Further investigations identify the mechanism of Fe;-mediated aggregation-induced enhancement of the POD-like activity and the inhibitory effect of captopril on the enhancement,which is highly dependent on their concentrations.Based on this finding,a colorimetric method for the detection of captopril is developed.This sensing approach exhibits the merits of simplicity,rapidness,reliability,and low cost,which has been successfully applied in quality control of captopril in pharmaceutical products.Moreover,the present sensing platform allows smartphone read-out,which has promising applications in point-of-care testing devices for clinical diagnosis and drug analysis.

Nanozyme Enhanced Colorimetric Immunoassay for Naked-Eye Detection of Salmonella Enteritidis

Salmonella Enteritidis is a major public health threat of global proportions,while it is still a serious challenge to develop point-of-care detection assay.In this study,nanozymes(Fe-MOF nanoparticles)were successfully synthesized and applied as signal in a colorimetric immunoassay for naked-eye detection of Salmonella Enteritidis.After optimization,the proposed assay was able to detect Salmonella Enteritidis with a detection limit of 34 CFU/mL.The coefficients of variation(CV)of the test were less than 7.0%after 30 days storage at 4°C.The estimated recoveries in milk samples of the colorimetric immu-noassay range from 94.68 to 124%,which indicated the developed method is capable of detecting Salmonella Enteritidis in real samples.This method provides a potential platform for Salmonella detection with naked eyes,which has a significant application value for foodborne pathogen analysis at the point-of-care.

Detection of biological thiols based on a colorimetric method

Biological thiols（biothiols）, an important kind of functional biomolecules, such as cysteine（Cys） and glutathione（GSH）, play vital roles in maintaining the stability of the intracellular environment. In past decades, studies have demonstrated that metabolic disorder of biothiols is related to many serious disease processes and will lead to extreme damage in human and numerous animals. We carried out a series of experiments to detect biothiols in biosamples, including bovine plasma and cell lysates of seven different cell lines based on a simple colorimetric method. In a typical test, the color of the test solution could gradually change from blue to colorless after the addition of biothiols. Based on the color change displayed, experimental results reveal that the percentage of biothiols in the embryonic fibroblast cell line is significantly higher than those in the other six cell lines, which provides the basis for the following biothiols-related study.

Single Particle Colorimetric Acid Phosphatase Activity Assay with CeO_(2)-modified Gold Nanoparticles

Acid phosphatase(ACP)is a ubiquitous phosphatase in living organisms.The abnormal variation of ACP is related to various diseases.Herein,we propose a colorimetric method based on CeO_(2)-modified gold core shell nanoparticles(Au@CeO_(2)NPs)to analyze ACP activity with high sensitivity and specificity.In this design,2-phospho-L-ascorbic acid trisodium salt(AAP)is dephosphorylated by ACP and produces reductive ascorbic acid(AA),which makes the CeO_(2)shell decomposition.A remarkable blue shift of localized surface plasmon resonance peak(LSPR,from yellow to green)along with the scattering intensity ratio changes from individual Au@CeO_(2)NPs are observed.ACP activity can be quantified by calculating the ratio changes of individual Au@CeO_(2)NPs.This assay reveals limit of detection(LOD)of 0.044 mU/mL and the linear range of 0.05–5.0 mU/mL,which are much lower than most of spectroscopic measurements in bulk solution.Furthermore,the recovery measurements in real samples are satisfactory and the capacity for practical application is demonstrated.As a consequence,Au@CeO_(2)NPs used in this assay will find new applications for the ultrasensitive detection of enzyme activity.

Nanomaterial Based Biosensors for Detection of Viruses Including SARS-CoV-2:A Review

The COVID-19 outbreak led to an uncontrollable situation and was later declared a global pandemic.RT-PCR is one of the reliable methods for the detection of COVID-19,but it requires transporting samples to sophisticated laboratories and takes a significant amount of time to amplify the viral genome.Therefore,there is an urgent need for a large-scale,rapid,specific,and portable detection kit.Nowadays nanomaterials-based detection technology has been developed and it showed advancement over the conventional methods in selectivity and sensitivity.This review aims at summarising some of the most promising nanomaterial-based sensing technologies for detecting SARS-CoV-2.Nanomaterials possess unique physical,chemical,electrical and optical properties,which can be exploited for the application in biosensors.Furthermore,nanomaterials work on the same scale as biological processes and can be easily functionalized with substrates of interest.These devices do not require extraordinary sophistication and are suitable for use by common individuals without high-tech laboratories.Electrochemical and colorimetric methods similar to glucometer and pregnancy test kits are discussed and reviewed as potential diagnostic devices for COVID-19.Other devices working on the principle of immune response and microarrays are also discussed as possible candidates.Nanomaterials such as metal nanoparticles,graphene,quantum dots,and CNTs enhance the limit of detection and accuracy of the biosensors to give spontaneous results.The challenges of industrial-scale production of these devices are also discussed.If mass production is successfully developed,these sensors can ramp up the testing to provide the accurate number of people aff ected by the virus,which is extremely critical in today’s scenario.

Palladium Nanoparticles/Graphdiyne Oxide Nanocomposite with Excellent Peroxidase-like Activity and Its Application for Glutathione Detection

In this study, palladium nanoparticles loaded graphdiyne oxide (Pd/GDYO) nanocomposite were fabricated by in-situ reduction of palladium chloride in the dispersion of GDYO, and characte-rized by Raman spectra, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The synthesized Pd/GDYO was first found to have catalytic activities similar to those of the peroxidase enzyme, which can catalyze the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine(TMB) in the presence of hydrogen peroxide(H2O2). Steady-state kinetic studies showed that the catalytic reaction of Pd/GDYO follows a ping-pong mechanism, and Pd/GDYO has a stronger activity to TMB with a Michaelis constant(Km) value of 5.32×10-4 mmol/L. Based on the shielding effect of glutathione(GSH) on the Pd/GDYO-H2O2-TMB reaction system, a colorimetric detection method for GSH was deve-loped with a wide linear range from 0.1 μmol/L to 40 μmol/L and a limit of detection of 0.1 μmol/L. The method was successfully applied for fast and accurate detection of GSH in injection powder drugs. It was expected that this peroxidase-like Pd/GDYO nano- composite would have wide applications in the fields of biomedicine and environmental chemistry.