Selective Colorimetric Method Research Articles

MOF-confined ultrafine nanozymes with enhanced catalysis for sensitive colorimetric detection of glucose

The constrained enzymatic activity and aggregation challenges encountered of nanozymes pose an obstacle to their practical utility, necessitating a strategy to alleviate this problem and improve the enzymatic catalytic efficiency. Herein, MOF-confined ultrafine ZnIrOx clusters (ZnIrOx/ZnIrMOFs) nanozyme have been synthesized by controlling the in-situ growth of ZnIrOx via a one-pot method. Notably, due to the domain-limiting effect of MOF, the ZnIrOx clusters with small size grew uniformly in the structure of ZnIrMOFs nanosheets, while the presence of Zn atoms improved the crystallinity of the MOF phase and the dispersion of ZnIrOx clusters. Moreover, the obtained ZnIrOx/ZnIrMOFs exhibited excellent oxidase- and peroxidase-like activities and better substrate affinity, which can directly and effectively catalyze chromogenic reaction to produce corresponding color and signal changes. Adopting this strategy, we established a visual, sensitive, and selective colorimetric method for glucose detection with a linear range and detection limit of 2.66–319 μM and 1.9 μM, respectively. The successful detection of glucose in real serum and food samples implies the promising MOF-confined ultrafine nanozymes in biomedical diagnostics and in food industry.

Developing oxygen vacancy-rich CuMn2O4/carbon dots dual-function nanozymes via Chan-Lam coupling reaction for the colorimetric/fluorescent determination of D-penicillamine

Defect engineering is a promising approach to construct high performance nanozymes due to its ability to regulate their physical and chemical properties. However, how to construct defects to improve the activity of nanozymes remains a challenge. Herein, for the first time, the Chan-Lam coupling reaction is used to construct the oxygen vacancy (OV)-rich CuMn2O4/carbon dots (CDs) (OV-CuMn2O4/CDs) dual-function nanozymes with fluorescent (FL) and oxidase-like properties, via regulating the low-valent metal ions (Cu+ and Mn2+) and Ov contents in the spinel CuMn2O4 and in-situ growth of β-cyclodextrin (β-CD)-derived CDs. Expectedly, relative to CuMn2O4, the OV-CuMn2O4/CDs exhibited 35.8%, 8.5%, and 14.6% rise in the contents of Cu+, Mn2+ and Ov, respectively. Abundant Ov provides more O2 adsorption/activation sites, and the charge transfer between Ov and metal atoms increases the charge density around metal atoms. This produces more low-valent metals (like Cu+ and Mn2+) to promote the electron transfer from metal to O atoms and O-O bond cleavage. Thus, the oxidase-like activity of OV-CuMn2O4/CDs is 4.1 times that of CuMn2O4. Also, the in-situ growth of β-CD-derived carbon dots on CuMn2O4 endows OV-CuMn2O4/CDs selective target recognition. Thus, a sensitive and selective colorimetric and fluorescence dual-mode method was established for determining D-penicillamine (D-PA), with the limit of detection of 0.25 and 0.048 μM, respectively. The method was applied to D-PA determination in real samples. This work demonstrates the Chan-Lam coupling reaction can be used to construct high performance nanozymes for developing dual-mode sensor for the detection of targets.

Mn‐functionalized ZIF‐67 for Sensitive Colorimetric Detection of Artemisinin and Ascorbic Acid

AbstractArtemisinin (Art) is the first‐choice medicine for treating malaria. It also shows good therapeutic effects in antibacterial and antitumor treatment. Therefore, a simple and efficient method for detecting Art is of great significance. Mn‐functionalized ZIF‐67 (Mn@ZIF‐67) hybrid materials were synthesized using a post‐synthesis modification method. Due to the introduction of Mn2+, Mn@ZIF‐67 possesses higher peroxidase/oxidase like dual‐mimic enzymes activity than ZIF‐67, which can catalyze Art and generate peroxyl radicals. Based on this fact, Mn@ZIF‐67 was used to detect Art in the presence of TMB. A good linear relationship between absorption intensities at 652 nm and Art concentrations was obtained in the range of 0–22 μM with a detection limit of 200 nM. Ascorbic acid (AA), due to its reducibility, can inhibit peroxidase/oxidase mimetics activity of Mn@ZIF‐67 and the blue Mn@ZIF‐67‐TMB solution fades gradually with the addition of AA. Mn@ZIF‐67‐TMB acting as a colorimetric sensor, AA can be sensitively detected ranging from 0–30 μM, and the detection limit is 50 nM. Coexisting substances don't interfere with the detection of Art or AA. This sensing platform provides a highly sensitive and selective colorimetric method for monitoring AA and Art in serum, thereby demonstrating remarkable practicality.

Casein/ferric chloride/polyvinyl alcohol composite for specific in-syringe colorimetric detection of formaldehyde in Hevea brasiliensis latex.

A new, simple, and selective colorimetric method of determining formaldehyde in Hevea brasiliensis latex was developed by using a casein/ferric chloride/polyvinyl alcohol hydrogel composite (casein/FeCl3/PVA) in a modified Leach test. Under heating, formaldehyde reacted with 8% casein in the presence of 0.1% FeCl3 and 4.3% HCl (v/v) entrapped in a 30% PVA hydrogel packed in a syringe. A purple-colored product was formed with a maximum absorbance of 525nm. The color change was evaluated at the color detection zone indicated on thethe syringe. The %magenta values were easily evaluated by using a mobile phone application and employed to determine formaldehyde content. The casein/FeCl3/PVA composite gave a readable response in a formaldehyde detection range from 0.04 to 0.80% with a linear response between %magenta and formaldehyde concentration (R2 = 0.9955). The detection limit was 0.032%, and precisions were in the range 0.67-4.94%. The casein/FeCl3/PVA composite was applied to the analysis of ammonia-preserved latex samples, and recoveries of formaldehyde from samples spiked at 0.1, 0.3, and 0.5% ranged from 81.55 to 99.51% (RSDs ≤ 5.41%). The recoveries and precision of the proposed method were comparable with those of high-performance liquid chromatography (HPLC). The developed method was also selective, showing no interference from other latex preservatives, i.e., phenol, ammonia, or tetramethylthiuram disulfide.

Colorimetric Detection of Sulfur Mustard with 4-(p-Nitrobenzyl)pyridine and Its Derivatives.

In this study, we present a simple, highly sensitive, and selective colorimetric method for detecting sulfur mustard (SM) and its simulants. This method relies on a nucleophilic substitution reaction between derivatives of 4-(p-nitrobenzyl)pyridine (NBP) and SM and subsequent treatment with an external base, resulting in a visible response. This reaction exhibits an impressively low detection threshold by the naked eye, as low as 10 ppm at room temperature. In contrast to the conventional use of NBP for detecting other alkylating agents, such as nitrogen mustard, our approach eliminates the need for prolonged heating or intricate extraction processes. Both computational and experimental investigations underscore the significance of water within our detection medium as it stabilizes crucial episulfonium cation intermediates. Furthermore, we demonstrate the practical applicability of this sensor by incorporating it onto cellulose and silica surfaces, which may provide guidance for the design and development of solid-state SM detectors.

Biosynthetic Gold Nanoparticles as Sensitive and Selective Colorimetric Method for Mercury Ions

Gold nanoparticles (AuNPs) are widely studied materials that have intense applications in various fields. Different methods use to synthesize the AuNPs, one of them is eco-friendly biological method, known as "green synthesis" a simple and inexpensive method. The Synthesis of AuNPs, using ginger extract is described here. The components found in ginger work as a stabilizing and reducing agent. Hydrogen tetrachloroaurate (HAuCl4) are mixed with the ginger extract to produce AuNPs. The Surface Plasmon Resonance (SPR) measured at λmax = 540 nm. UV-Vis, FTIR and Zeta potential are used to identify AuNPs, as well as AFM and SEM. Data indicate that AuNPs has a spherical structure. The results demonstrate that AuNPs stabilized by ascorbic acid were used as a colorimetric probe for Hg2+ ions, based on the production of (Au-Hg) amalgamate which made changes in their absorbance, due to its ability to reduce Hg2+ to Hg0, to enhance amalgamation. Practically, this procedure has successfully worked to detect Hg2+ in tap water as a sensitive and selective probe with upper limit of detection equal to 120 µM as well as the limit of detection (LOD) equal to 0.65 µM, so the method proved to be sensitive and selective probe.

Colorimetric strategy of Fe/Co-doped porphyrin metal-organic frameworks with enzyme-like activity for the detection of ascorbic acid

The multifunctional structures and functions of metal-organic frameworks with enzyme-like activity make them promising for various applications. This study has shown Fe/Co-doped porphyrin metal-organic framework (PCN-224-Fe/Co) (PCN-224 = porous coordination network-224) efficiently catalyzed the substrates hydrogen peroxide (H2O2) and 3,3′,5,5′-tetramylbenzidine (TMB) to generate oxidized-TMB (oxTMB). The results of the kinetic experiments showed that PCN-224-Fe/Co had a greater affinity for TMB compared to PCN-224-Fe or PCN-224-Co. A selective colorimetric method for determining ascorbic acid (AA) was also successfully developed due to the peroxidase-like activity of PCN-224-Fe/Co. The linear concentration-response range was found to be between 1 and 200 μM with a detection limit of 0.2 μM. This research will facilitate the creation of peroxidase mimics utilizing porphyrin metal-organic frameworks for use in detective applications.

A highly sensitive method for the detection of p-Aminophenol based on Cu–Au nanoparticles and KIO3

BackgroundAs a common industrial raw material and chemical intermediate, p-Aminophenol (pAP) is recognized as a serious pollutant that poses harm to both the environment and human health. The traditional detection methods for pAP have the advantages of good selectivity and high sensitivity, but their complex operation and time-consuming defects limit their application in on-site detection. Therefore, it is necessary to develop a simple, low-cost, rapid and high-sensitivity method for the detection of pAP. ResultsNoble metal nanoparticles have been widely used in colorimetric sensing because of their simplicity and practicality. Herein, we presented a simple, excellent sensitive and selective colorimetric method for high-performance detection of pAP based on Cu–Au nanoparticles (Cu–Au NPs) and KIO3. In the presence of pAP, KIO3 was reduced to I2, which subsequently chemically adsorbed onto Cu–Au NPs surface and induced the dispersion and reorganization of Cu–Au NPs, along with prominent color change of the dispersion from gray-blue to pink and the transformation of Cu–Au NPs from chain-like aggregates to individual dispersed, irregular, subspherical nanoparticles. The mechanism was verified by TEM, DLS, Zeta potential, UV–vis and XPS. Meanwhile, Cu–Au NPs probe can rapidly detect pAP within 25 min, the limit of detection of pAP probe is 5 μM by the naked eyes and 0.03 μM by UV–vis absorption spectrum. Significance and noveltyThis is the first colorimetric assay for pAP based on Cu–Au NPs probe. The satisfactory linearity (R2 = 0.9984) indicates that the colorimetric probe based on Cu–Au NPs and KIO3 can be utilized for quantitative detection of pAP. The detection results of pAP in real environmental water samples, urine samples and paracetamol tables demonstrate the practicability of pAP colorimetric probe.

Ultra-trace level colorimetric composite sensor based on novel DH-1,6-NAPY-8-CN-AgNPs for the detection of Clonazepam in aqueous and human plasma samples

It’s crucial to develop cost-effective, rapid & reliable detection methods of specific analytes through sensors useful in the analytical and medicinal chemistry. We report a highly sensitive and selective colorimetric method for the detection of clonazepam by the new composite sensor DH-1,6-NAPY-8-CN-AgNPs containing 5-amino-7-(4-benzylpiperazin-1-yl)-2,4-bis(4-bromophenyl)-2-methyl-1,3-dihydro-1,6-naphthyridine-8-carbonitrile capped silver nanoparticles (AgNPs). The detection mechanism is based on the H-bonding interactions between clonazepam and the sensor, that caused the aggregation of NPs that initiated a sharp color change from yellow to red. The linear relationship between the adjacent absorbance values (ΔA) vs. clonazepam concentration (range = 0.05–75 µM) showed a correlation coefficient of 0.9927. The limit of detection (LOD) was 3 nM, that is very significant achievement over the existing reports. The proposed sensor is highly selective, with no interference from many other possible interfering substances. The sensor was successfully applied to the aqueous and human plasma samples, therefore DH-1,6-NAPY-8-CN-AgNPs demonstrated great potential for the on-site and real-time screening of Clonazepam.

Detection of silver ions by a novel NBD-based colorimetric chemosensor with the intramolecular charge transfer character

Development of a selective colorimetric method for Ag+ is important because excessive accumulation of Ag+ in the environment or human body causes significant damage. Therefore, a novel NBD-based colorimetric chemosensor NPB (7-nitro-N-(2-phenoxyphenyl)benzo[c][1,2,5]oxadiazol-4-amine) was designed for detecting silver ions. NPB could detect Ag+ with a distinctive bathochromic shift in absorption spectra and color change from pale brown to pink. The stoichiometric binding ratio between NPB and Ag+ was confirmed to be a 1:1 by Job plot analysis. The detection limit of Ag+ was found to be 3.60 × 10−6 M. Importantly, NPB showed an improved intramolecular charge transfer (ICT) in complexation with Ag+, which was demonstrated through DFT calculations. The ICT process change was in good agreement with the experimental results of the spectroscopic response. In addition, NPB could be applied to the detection of Ag+ through a change of color in test kits.

A novel chromogenic agent for colorimetric estimation of copper in water and pharmaceutical tablets

The current study intends to develop a novel, simple and selective colorimetric method to identify the presence of copper on the basis of a newly-synthesized Schiff base prepared from the condensation of 1,3-phthalaldehyde with 2-aminophenol. The new chromogenic agent was studied using elemental analysis, IR, NMR, MS, and UV-VIS absorption. The new chelate produces a stable colored complex with Cu(II) ions with a 1:1 composition using the continuous variation method. The colored complex exhibited a strong absorption peak at 440 nm and this phenomenon empowers copper analysis to be conducted in natural water, and pharmaceutical solutions directly via the colorimetric method. The author analyzed and optimized all the factors that tend to influence the sensitivity of the method. Such factors include pH, time, the concentration of the ligand and temperature. The process, proposed in the study, was then evaluated in terms of precision, accuracy and selectivity. The study outcomes are as follows: linear dynamic range 0.8-22 ug mL-1; limit of quantitation 0.970 ug mL-1; and limit of detection 0.293 ug mL-1.

A Sensitive and Selective Colorimetric Method Based on the Acetylcholinesterase-like Activity of Zeolitic Imidazolate Framework-8 and Its Applications.

In this study, a simple colorimetric method was established to detect copper ion (Cu2+), sulfathiazole (ST), and glucose based on the acetylcholinesterase (AChE)-like activity of zeolitic imidazolate framework-8 (ZIF-8). The AChE-like activity of ZIF-8 can hydrolyze acetylthiocholine chloride (ATCh) to thiocholine (TCh), which will further react with 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) to generate 2-nitro-5-thiobenzoic acid (TNB) that has a maximum absorption peak at 405 nm. The effects of different reaction conditions (buffer pH, the volume of ZIF-8, reaction temperature and time, and ATCh concentration) were investigated. Under the optimized conditions, the value of the Michaelis-Menten constant (Km) is measured to be 0.83 mM, which shows a high affinity toward the substrate (ATCh). Meanwhile, the ZIF-8 has good storage stability, which can maintain more than 80.0% of its initial activity after 30 days of storage at room temperature, and the relative standard deviation (RSD) of batch-to-batch (n = 3) is 5.1%. The linear dependences are obtained based on the AChE-like activity of ZIF-8 for the detection of Cu2+, ST, and glucose in the ranges of 0.021-1.34 and 5.38-689.66 µM, 43.10-517.24 µM, and 0.0054-1.40 mM, respectively. The limit of detections (LODs) are calculated to be 20.00 nM, 9.25 µM, and 5.24 µM, respectively. Moreover, the sample spiked recoveries of Cu2+ in lake water, ST in milk, and glucose in strawberry samples were measured, and the results are in the range of 98.4-115.4% with the RSD (n = 3) lower than 3.3%. In addition, the method shows high selectivity in the real sample analysis.

A new selective colorimetric method coupled with a high-resolution UV method for the consecutive quantification of three drugs in semi-solid preparations

Triderm® cream and ointment contain clotrimazole (CLO), betamethasone dipropionate (BET), and the poor UV absorbing gentamycin (GEN), in addition to the preservative benzyl alcohol (BEN) which exists only in a cream preparation. A green, selective colorimetric approach was elaborated to increase the sensitivity of GEN quantification in Triderm® preparations, which depends on the immediate formation of a pink ion-pair between GEN and erythrosine (ERY) reagent in an aqueous acidic medium. The ion pair was made soluble in water with the assistance of the surfactant agent poloxamer 188 which is presented in this manuscript as an efficient solubilizing agent for the hydrophobic ion-pair. This surfactant agent has the feature of not affecting the native color of ERY, additionally the ease of preparing its aqueous solution with no need for heating or long waiting. The resulting complex GEN-ERY was measured directly at 545nm. This colorimetric approach was coupled with the Unlimited Derivative Ratio (UDD), which is a new smart UV method employed for the concurrent quantification of BET and CLO in Triderm® preparations without any intervention from BEN, due to its capability to resolve an extremely overlapped ternary spectrum that has no extended part, iso-absorptive point or robust zero crossing point. The newly developed UDD method depends on filtrating and measuring the signal of BET and CLO through calculating the equality factor(F) for CLO and BET after dividing their spectrum by BEN spectrum, derivatizing the resulting ratio spectrum, then constructing a regression equation employing the F factor for each BET and CLO. The overlapping excipient BEN was quantified via the Double Divisor Ratio spectra derivative method (DDR) relying on using a divisor comprising of a mix of BET + CLO. The advanced spectrophotometric approach validity was checked by confirming the linearity, accuracy, precision, and specificity in accordance with the ICH directions. NO notable difference when statistically comparing the newly established approach to the reference approach.

L-cysteine-regulated in situ formation of Prussian blue/Turnbull’s blue nanoparticles as the colorimetric probe for the detection of copper ion

A fast, simple, sensitive, and selective colorimetric method for the detection of Cu 2+ was developed using Prussian blue/Turnbull’s blue nanoparticles (PBNPs/TBNPs) as the probe. The colorimetric sensor is based on the following principle. Cu 2+ can induce the aggregation of L-cysteine (L-cys) modified-PBNPs/TBNPs (L-cys-PBNPs/TBNPs), resulting in an obvious red shift of its maximum absorption peak. Thus, the concentration of Cu 2+ can be determined based on the peak shift in the UV–Vis spectra. The optimal pH, concentration of L-cys, reaction temperature between L-cys-PBNPs/TBNPs and Cu 2+ , the formation time of L-cys-PBNPs/TBNPs, and the reaction time between L-cys-PBNPs/TBNPs and Cu 2+ of the method were determined to be pH 4.5, 2.0 mM, 20 °C, 5.0 min, and 2.0 min, respectively. A good linearity for the colorimetric determination of Cu 2+ at the range of 0.25–2.5 μM (R 2 = 0.986) was obtained, with a limit of detection (LOD) of 0.12 μM. Moreover, the negligible response of other metal ions demonstrates good selectivity and specificity of the sensor. In addition, the method was employed in the detection of Cu 2+ in lake water samples, and the spiked recoveries are in the range of 96.7–106.6% with a relative standard deviation less than 7.4%. Therefore, the colorimetric method is applicable for Cu 2+ detection in real water samples of high sensitivity and selectivity.

Metal-doped carbon dots as peroxidase mimic for hydrogen peroxide and glucose detection.

Carbon dots (CDs) have several superior characteristics including sufficient carbon sources, easy preparation, no toxicity, and high catalytic efficiency as a new kind of nanozyme. Herein, Ce-doped carbon dots (Ce-CDs), Cr-doped carbon dots (Cr-CDs), Cu-doped carbon dots (Cu-CDs), Fe-doped carbon dots (Fe-CDs), Mn-doped carbon dots (Mn-CDs), and non-metal-doped carbon dots (0-CDs) were synthesized to explore the detection of hydrogen peroxide (H2O2) and glucose as peroxidase mimic. The prepared CDs could efficiently oxidize the colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxTMB in the presence of H2O2. After adding glucose oxidase (GOD) to the CDs/TMB system, a colorimetric method for glucose detection was developed. The results show that Fe-CDs possess the highest catalytic activity. When using Fe-CDs as peroxidase mimetics, the detection limit of this assay for glucose was 0.029mmol L-1. This successfully provides a sensitive and selective colorimetric method for hydrogen peroxide and glucose determination.

Cu-MOF@Pt 3D nanocomposites prepared by one-step wrapping method with peroxidase-like activity for colorimetric detection of glucose

As an alternative to natural enzymes, artificial enzymes based on nanomaterials have attracted a lot of attention owing to their outstanding catalytic activity and high stability as well as low cost. Cu-MOF loaded with platinum nanoparticles (labeled Cu-MOF@Pt) was prepared by simple one-step wrapping method using platinum nanoparticles, copper nitrate trihydrate and 1,3,5-tricarboxybenzene. It was confirmed that Cu-MOF@Pt exhibits peroxidase-like activity, which can quickly catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) and produce blue oxidized TMB (oxTMB) in the presence of hydrogen peroxide (H2O2). Additionally, steady-state kinetics showed that Cu-MOF@Pt exhibits stronger appetency to TMB and H2O2 compared with horseradish peroxidase. Thanks to the peroxidase-like activity of Cu-MOF@Pt, a highly selective colorimetric method for glucose detection has been successfully established, the linear range is 2–15 mM (R2 =0.9999) and the Limit of Detection (LOD) is 0.42 mM, with a detection range that meets clinical needs. Moreover, its good intra- and inter-assay precision and excellent stability make the results of glucose detection very reproducible. The detection performance of 90.09% was still maintained at 4 ℃ for 2 months. In conclusion, a new nanocomposite was successfully prepared and its selective detection ability for glucose was proved, which established a good basis for the clinical development of new enzymes for biosensors.

Fe single atoms anchored on fluorine-doped ultrathin carbon nanosheets for sensitive colorimetric detection of p-phenylenediamine

Single-atom catalysts have attracted enormous research interest in the field of catalysis owing to their remarkable catalytic activity, excellent stability and outstanding atom utilization. Herein, a new single atom based on single Fe atoms on fluorine-doped (Fe–SAs@FNC) ultrathin carbon nanosheets was successfully synthesized by a polymer-assisted heating method. Experimental evidence showed that the resultant Fe–SAs@FNC with Fe–N4 sites exhibits superior peroxidase-like activity, which oxidizes the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to produce a blue product in the presence of hydrogen peroxide (H2O2). Based on this, an ultrasensitive and highly selective colorimetric detection method for p-phenylenediamine (PPD) in hair dyes and PPD in hair after dyeing was established, which had a wide linear range (0.2–50 μM) and low detection limit (0.07 μM). This method shows satisfactory sensitivity and selectivity.

Lycium Barbarum Polysaccharide-Iron (III) Chelate as Peroxidase Mimics for Total Antioxidant Capacity Assay of Fruit and Vegetable Food.

Quantitative evaluation of the antioxidant capacity of foods is of great significance for estimating food’s nutritional value and preventing oxidative changes in food. Herein, we demonstrated an easy and selective colorimetric method for the total antioxidant capacity (TAC) assay based on 3,3’,5,5’-tetramethyl-benzidine (TMB), hydrogen peroxide (H2O2) and synthetic Lycium barbarum polysaccharide-iron (III) chelate (LBPIC) with high peroxidase (POD)-like activity. The results of steady-state kinetics study showed that the Km values of LBPIC toward H2O2 and TMB were 5.54 mM and 0.16 mM, respectively. The detection parameters were optimized, and the linear interval and limit of detection (LOD) were determined to be 2–100 μM and 1.51 μM, respectively. Additionally, a subsequent study of the determination of TAC in six commercial fruit and vegetable beverages using the established method was successfully carried out. The results implied an expanded application of polysaccharide-iron (III) chelates with enzymatic activity in food antioxidant analysis and other biosensing fields.

Bifunctional nanozyme of copper organophyllosilicate for the ultrasensitive detection of hydroquinone.

The rapid development of nanozymes for ultrasensitive detection of contaminate has resulted in considerable attention. Herein, a carboxyl- and aminopropyl-functionalized copper organophyllosilicate (Cu-CAP) was synthesized by a facile, one-pot sol-gel method. The bifunctional groups endow it with superior catalytic activity than that of natural enzyme. Besides, it possesses outstanding catalytic stability under harsh conditions such as high temperature, extremely high or low pH, and high salinity. Apart from laccase-mimetic activity, Cu-CAP also shows oxidation of the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) to the blue-colored TMBox in the presence of H2O2, which is similar to natural horseradish peroxidase (HRP). Interestingly, this colorimetric system was suppressed by hydroquinone (HQ) specifically. Inspired by this, Cu-CAP was used to develop a highly sensitive and selective colorimetric method for the determination of HQ. This assay displayed an extremely low detection limit of 23nM and was applied for the detection of HQ in environmental water with high accuracy. This approach offers a new route for the rational design of high performance nanozymes for environmental and biosensing applications.

Colorimetric detection of Hg2+ ion using fluorescein/thiourea sensor as a receptor in aqueous medium

A simple and selective colorimetric method was used for detection of mercury ions (Hg2+) in the aqueous medium and environmental water sample. Fluorescein-thiourea (FLTU) conjugate complex as receptor was prepared by 1:1 M ratio of 10 µmol L−1 fluorescein dye and thiourea in phosphate buffer solution at pH 7.00. The IR spectra of FLTU receptor have illustrated the formation of H-bond between thiourea and fluorescein dye. The receptor fluorescein-thiourea (FLTU) was used as a chemo-sensor highly selectively to detect the Hg2+ ion by changing the receptor color from a yellow color to red through the quenching of fluorescence intensity of the receptor. The lower detection limit (LOD) and quantitation limit (LOQ) of Hg2+ ions with the receptor FLTU are 0.24 and 0.73 nmol L–1 (correlation coefficient R2 = 0.985) as determined through the absorption spectroscopic method, respectively. The mercury ions are promising and could be achieved via the formation of the complex in a 1:1 stoichiometric ratio of receptor to Hg2+ ions.