Label-free Colorimetric Sensor Research Articles

3,3′,5,5′-Tetramethylbenzidine assisted construction of a label-free colorimetric and ratiometric fluorescent dual-channel sensor for the detection of levofloxacin

A colorimetric and ratiometric fluorescent dual-channel sensor was constructed for levofloxacin (LVFX) detection only using 3,3′,5,5′-tetramethylbenzidine (TMB). LVFX was found to be a light-activatable oxidase mimic, which can generate free radicals and oxidize colorless TMB to blue product (oxTMB) upon irradiation at 365 nm, providing a visible color signal. Interestingly, increasing LVFX concentration causes more TMB to be oxidized, leading to a reduction in the fluorescence intensity of TMB at 410 nm and a rise in the fluorescence intensity of LVFX at 511 nm under excitation at 350 nm. The detection limits of the colorimetric and ratiometric fluorescent sensor for LVFX were 0.26 μM and 0.23 μM, respectively. Moreover, a smartphone-based dual-channel portable device was constructed to integrate the photocatalytic oxidation reaction and the colorimetric and fluorescent detection of LVFX. Successfully detecting LVFX in milk and egg white samples in 5 min suggests the device has great potential in on-site assays.

Label-free colorimetric sensor for Pb2+ determination using catalytic activity of MnO2 nanoflowers and elongated aptamer

In this study, a label-free aptasensor utilizing colorimetric properties was developed to detect Pb2+ with high sensitivity. The approach involved applying modified aptamer which enhanced the oxidase-mimicking activity of MnO2 nanoflowers. This innovative method provides an efficient means for monitoring Pb2+ ions without requiring any labeling techniques. The fundamental principle of this aptasensor is based on the adsorption of a modified aptamer onto MnO2 nanoflowers' surface, which in turn increases their affinity for chromogenic substrates and enhances their catalytic activity. The proposed aptasensor exploits the high sensitivity due to the extension of the aptamer sequence length by terminal deoxynucleotidyl transferase (TdT). Under optimum experimental conditions, the developed colorimetric aptasensor indicated a linear detection range from 4 to 80 nM with a limit of detection (LOD) of 1.4 nM. Moreover, the aptasensor successfully monitored Pb2+ in the drinking water, milk and human serum samples. Henceforth, the colorimetric aptasensor exhibited in this study possesses several benefits such as uncomplicated operation, cost-effectiveness, label-free detection and remarkable sensitivity. Thus rendering it a suitable option for analyzing intricate samples.

Engineering constructed of high selectivity dexamethasone aptamer based on truncation and mutation technology.

Various biosensors based on aptamers are currently the most popular rapid detection approaches, but the performance of these sensors is closely related to the affinity of aptamers. In this work, a strategy for constructed high-affinity aptamer was proposed. By truncating the bases flanking the 59 nt dexamethasones (DEX) original aptamer sequence to improve the sensitivity of the aptamer to DEX, and then base mutation was introduced to further improve the sensitivity and selectivity of aptamers. Finally, the 33 nt aptamer Apt-M13 with G-quadruplex structures was obtained. The dissociation constant (Kd) was determined to be 200 nM by Graphene oxide (GO)-based fluorometry. As-prepared Apt-M13 was used for a label-free colorimetric aptamer sensor based on gold nanoparticles, the LOD was 3.2-fold lower than the original aptamer described in previous works. The anti-interference ability of DEX analogs is also further improved. It indicates that truncation technology effectively improves the specificity of the aptamer to DEX in this work, and the introduction of mutation further improves the affinity and selectivity of the aptamer to DEX. Therefore, the proposed aptamer optimization method is also expected to become a general strategy for various aptamer sequences.

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 robust colorimetric aptasensor for the label-free detection of marine toxins based on tyrosine-capped gold nanoparticles.

PbTx-2 and okadaic acid (OA) are two typical marine toxins that are highly toxic and harmful to human health. The approach based on citrate-capped gold nanoparticles (Cit-AuNPs) and specific aptamers to construct label-free colorimetric sensors is a widely used method for marine toxin detection. However, the potential interactions between Cit-AuNPs and target molecules have always been ignored, which may result in wrong analytical results due to shortcomings in the Cit-AuNPs. To overcome these shortfalls, in this work, AuNPs were synthesized using tyrosine as a reducing and capping agent, and a robust colorimetric aptasensor based on tyrosine-capped AuNPs (Tyr-AuNPs) was constructed for the label-free detection of marine toxins. Tyr-AuNPs presented better stability compared to Cit-AuNPs due to the stronger binding of amine groups on tyrosine to AuNPs through the Au-N bond. Interactions between Tyr-AuNPs and PbTx-2 were analyzed through UV-vis and isothermal titration calorimetry methods and the results validated the robustness of the Tyr-AuNPs. Colorimetric aptasensors were established for PbTx-2 and OA detection with a linear range of 0.05-4 ppm and limits of detection of 2.25 ppb and 5.19 ppb, respectively. These results demonstrate that the developed colorimetric aptasensor can be a robust and promising method for marine toxin detection.

Dissecting the Effect of Salt for More Sensitive Label-Free Colorimetric Detection of DNA Using Gold Nanoparticles.

Taking advantage of the protection effect of single-stranded DNA oligonucleotides, gold nanoparticles (AuNPs) remain dispersed and retain a red color with the addition of a low concentration of salt, while AuNPs would aggregate in the presence of double-stranded DNA. This difference has been used to design label-free colorimetric sensors for DNA detection. NaCl is the most commonly used salt to induce the aggregation of AuNPs. In this work, we aimed to test if other salts can provide even better sensor performance and to understand the effects of the cations and anions in salts. We first studied the effect of anions, including halides (NaF, NaCl, NaBr, and NaI), and other common salts (NaNO3, NaClO4, Na2SO4, Na2S2O3, sodium phosphate, and sodium citrate). Among them, weakly adsorbing ones such as F-, citrate, and phosphate appeared to yield better sensitivity than Cl-. Anions can directly adsorb on the AuNPs and affect DNA adsorption. We then tested cations, and only group 1A metals (LiCl, NaCl, KCl, RbCl, and CsCl) can signal DNA adsorption, while divalent metals (MgCl2, CaCl2, MnCl2, and NiCl2) barely showed the effect of DNA. CsCl only works for strongly adsorbing DNA, such as A15, but not weakly adsorbing T15. Overall, NaF is a better salt than NaCl by having a 2.3-fold higher sensitivity, which was confirmed in a DNA sensing assay. This work has identified a better salt yielding higher sensitivity, and sensing work relying on the change of the aggregation state of AuNPs can benefit from this study.

Distributed Bragg reflectors for the colorimetric detection of bacterial contaminants and pollutants for food quality control

Real-time monitoring of bacterial contaminants and pollutants in food is of paramount importance nowadays, owing to the impressive extension of the food production/supply chain and the consequent increase in foodborne outbreaks worldwide. This represents a serious risk for consumers’ health and accounts for a large fraction of food wastage, especially in the developed countries. Therefore, modern sensors for food quality control should possibly afford low-cost, portability, and easiness of readout to enable widespread diffusion of the technology, thus allowing food quality monitoring from the production/supply chain to the consumers’ table. In these regards, one-dimensional photonic crystals, also known as Distributed Bragg Reflectors (DBRs), can represent simple yet efficient all-optical and label-free colorimetric sensors, given their relatively high color purity, easiness of integration with a large number of stimulus responsive materials, and low-cost fabrication from scalable processes. In this perspective article, we discuss the development of DBRs-based colorimetric sensors for the monitoring of bacterial contaminants and pollutants of interest in the food quality sector. We aim at providing a systematic overview on the main approaches that have been employed to achieve selectivity and sensitivity in DBRs-based sensors, with the view to enable widespread use of this technology at both the industry/supply chain and customers’ level.

Flory-Huggins Photonic Sensors for the Optical Assessment of Molecular Diffusion Coefficients in Polymers.

The lack of cost-effective systems for the assessment of air pollutants is a concern for health and safety in urban and industrial areas. The use of polymer thin films as label-free colorimetric sensors featuring specific interactions with pollutants would then represent a paradigm shift in environmental monitoring and packaging technologies, allowing to assess air quality, formation of byproducts in closed environment, and the barrier properties of the polymers. To this end, all-polymer distributed Bragg reflectors represent a promising approach toward a reliable and cost-effective transduction of chemical stimuli and effective colorimetric label-free selective detectors. We show selectivity attained by specific interactions between the polymer and analytes. Such interactions drive the analyte intercalation through the polymer structure and its kinetics, converting it in a dynamic optical response which is at the basis of the Flory-Huggins photonic sensors. The multivariate analyses of the response kinetics also allow distinguishing binary mixtures. Additionally, we demonstrate that such optical responses can be used to esteem the diffusion coefficients of small molecules within polymer media via simple UV-vis spectroscopy retrieving data comparable to those obtained with state-of-the-art gravimetric procedures. Last, we assess the figures of merit of the sensors in terms of lower detection limit, sensitivity, and reversibility, demonstrating that such devices can pave the way to an innovative, simple, and low-cost detection method integrable to in situ assessment of barrier polymers used for the encapsulation of optoelectronic devices, food packaging, and goods storage in general.

Sensitive Colorimetric Hg2+ Detection via Amalgamation-Mediated Shape Transition of Gold Nanostars.

Reliable and sensitive methods to monitor mercury levels in real samples are highly important for environment protection and human health. Herein, a label-free colorimetric sensor for Hg2+ quantitation using gold nanostar (GNS) has been demonstrated, based on the formation of Au-Hg amalgamate that leads to shape-evolution of the GNS and changes in its absorbance. Addition of ascorbic acid (AA) to GNS solution is important for quantitation of Hg2+, mainly because it can reduce Hg2+ to Hg to enhance amalgamation on the GNSs and stabilize GNSs. In addition to transmission electron microscopy images, the distribution of circular ratios of GNSs in the presence of 2 mM AA and various concentrations of Hg2+ are used to show the morphology changes of the GNSs. Upon increasing the concentration of Hg2+, the average circular ratio of GNSs decreases, proving GNS is approaching to sphere. The morphology change alters the longitudinal localized surface plasmonic resonance (LSPR) absorbance of the GNSs significantly. Under the optimum conditions, our sensor exhibits a dynamic response for Hg2+ in the range of 1–4,000 nM with a detection limit of 0.24 nM. Upon Increasing Hg2+ concentration, the solution color changes from greenish-blue, purple to red, which can be distinguished by the naked eye when the Hg2+ concentration is higher than 250 nM. Owing to having a high surface-to-volume ratio and affinity toward Hg0, the GNS is sensitive and selective (at least 50-fold over tested metal ions like Pb2+) toward Hg2+ in the presence of AA. Practicality of this assay has been validated by the analysis of water samples without conducting tedious sample pretreatment.

Label-free colorimetric sensor for sensitive detection of choline based on DNAzyme-choline oxidase coupling

Changes in choline levels can be associated with diseases such as Alzheimer, Parkinson, Huntington, fatty liver, interstitial lung abnormalities, autism and so on. Therefore, quantitative determination of choline is important in the biological and clinical analysis. So far, several methods have been investigated for measuring choline in the body fluids, each of which has disadvantages such as the need for specialist ability, complexity, and high cost. For this purpose, a facile and sensitive colorimetric biosensor based on DNAzyme-choline oxidase coupling used for the determination of choline. In this method, the first, choline oxidase produces H2O2 and betaine in the presence of choline and oxygen, then, the DNAzyme converts colorless ABTS into green ABTS+ radicals. Compared to the previous methods, the linear range and the limit of detection of this talented biosensor were 0.1–25 μM and 22 nM. Choline measurement using this biosensor has shown satisfactory selectivity and repeatability. Its recovery was 96.9-103.7%, which shows the reliability of biosensor assay in biological samples. Simplicity, low cost, naked eye, high sensitivity, and precision are the benefits of this biosensor. Taken to gather, the proposed system can be considered as a great biosensor for measuring choline levels especially in point of care diagnostic.

A label-free colorimetric sensor based on silver nanoparticles directed to hydrogen peroxide and glucose

A simple method has been developed for preparation of silver nanoparticles (AgNPs) based on the use of graphene quantum dots (GQDs) as a reducing agent and a stabilizer. The synthesized nanocomposites consisting of silver nanoparticles and graphene quantum dots (AgNPs/GQDs) has been characterized by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Ultraviolet–visible spectroscopy (UV–Vis), Fourier-Transform Infrared spectroscopy (FT-IR), Energy Dispersive X-ray spectroscopy (EDX) and Dynamic Light Scattering (DLS). Results indicate that monodisperse of AgNPs has been obtained with particles size ca. ∼ 40 nm and specific plasmon peak of silver nanoparticles at 425 nm by UV–Vis spectrum. Using AgNPs/GQDs nanocomposite, we have constructed a colorimetric sensor for hydrogen peroxide (H2O2) and glucose sensors based on the use of AgNPs/GQDs as both probes: capture probe and signal probe. The fabricated sensors perform good sensitivity and selectivity with a low detection limit of 162 nM and 30 μM for H2O2 and glucose sensing, respectively. Moreover, the biosensors have been successfully applied to detect glucose concentrations in human urine.

Experimental and theoretical approaches for the selective detection of thymine in real samples using gold nanoparticles as a biochemical sensor

We report a simple, selective and cost effective method for the qualitative and quantitative determination of thymine in a DNA standard and urine samples using gold nanoparticles (AuNPs) as a label-free colorimetric biochemical sensor. The mechanism for the detection of thymine is demonstrated via the color change of the AuNPs from pink to blue, followed by the shift of the localized surface plasmon resonance (LSPR) absorption band to a higher wavelength with the introduction of an analyte. The selective detection of thymine was experimentally verified by performing a control experiment with nucleobases, other biomolecules, metal ions and anions. In addition, the computation density functional theory (DFT) and time dependent density functional theory (TD-DFT) using the Gaussian (C.01) program highlighted that the electrostatic potential behavior of the thymine molecule facilitated a non-covalent interaction toward gold for the selective detection of analytes, and the computation was also used to calculate a UV-Vis absorption band as well. The calculated absorption band of the AuNPs with thymine, obtained using TD-DFT, was found to be very close to the experimental data. The omnicapped truncated tetrahedral (ν3-tetrahedral) Au20 cluster structure was considered as the model for the AuNP optimization. The linear range obtained for the quantitative determination of thymine was found to be 10–1200 ng mL−1 with a limit of detection of 3 ng mL−1. The advantages of using the AuNPs as a biochemical sensor are that they provide a facile and low cost method and are selective for the qualitative and quantitative determination of thymine in a DNA standard and in urine samples in comparison to chromatographic and electrochemical methods.

Graphitic carbon nitride supported platinum nanocomposites for rapid and sensitive colorimetric detection of mercury ions

In this study, graphitic carbon nitride supported platinum nanocomposites (g-C3N4/PtNPs) have been synthesized for the first time by an ultrasonic-assisted chemical reduction method. By using g-C3N4 as the stabilizer, Pt ions could be reduced to PtNPs by NaBH4 and uniformly deposited on the surface of g-C3N4. The resulting g-C3N4/PtNPs exhibited enhanced catalytic activity for the oxidation of 3,3′,5,5’-tetramethylbenzidine (TMB) compared to that of g-C3N4 or PtNPs alone. After interaction with Hg2+, the catalytic activity of g-C3N4/PtNPs was effectively inhibited due to the formation of HgPt amalgam. On the basis of this effect, a novel label free colorimetric sensor has been developed for sensitive detection of Hg2+ through the g-C3N4/PtNPs mediated catalytic reaction. A detection limit as low as 1.23 nM was achieved. This assay also exhibited excellent selectivity toward Hg2+ over other metal ions. In addition, it was successfully applied to the determination of Hg2+ in real water samples. In view of the advantages, such as simple operation, cost-effective, rapid response and naked-eye observation, the developed colorimetric sensor hold great potential for the detection of toxic Hg2+ in environmental and biological samples.

Colorimetric detection of manganese(II) ions using alginate-stabilized silver nanoparticles

Marine algal polysaccharides have been recognized as the most effective and promising substrates for reduction and stabilization of metal nanoparticles. Ecofriendly synthesis of highly stable silver nanoparticles using alginate, an algal polysaccharide, is described herein. These alginate-stabilized silver nanoparticles (Alg-AgNPs) act as a label-free colorimetric sensor for quantification of manganese(II) metal ions (Mn2+) with excellent selectivity and sensitivity and detection in aqueous solution in the range of 1–10 µM. On addition of Mn2+ ions, the binding forces between Alg-AgNPs and Mn2+ ions bring the silver nanoparticles closer, decreasing the interparticle distance and causing slight agglomeration, with a color change from pale yellow to brownish yellow. Transmission electron microscopy images of Alg-AgNPs in presence of Mn2+ ions showed aggregation of AgNPs. Photon correlation spectroscopy showed decreases in the surface charge of Alg-AgNPs in presence of Mn2+ ions, thus predicting cross-linking aggregation. The ratio of absorbance at 500 nm to 400 nm (A 500/A 400) versus the Mn2+ ion concentration was linear with calibration curve of A 500/A 400 = 0.1611 + 0.0201 × C Mn. Such selective and sensitive detection of Mn2+ ions over other analytes appears suitable for application in point-of-use analysis of aqueous environmental samples.

From boron clusters to gold clusters: new label-free colorimetric sensors.

A fundamental boron cluster, [closo-B12H12]2-, mainly known as an inert agent for boron neutron capture therapy, can also serve as a bi-functional reductant and a capping agent for the formation of monodispersed size-controlled gold nanoparticles (AuNPs) under mild reaction conditions. The obtained AuNPs remained thermo-dynamically stable over 3 months, and exhibited high performance in phase transfer and sensing of heavy metal ions.

Enhancing sensitivity and selectivity in a label-free colorimetric sensor for detection of iron(II) ions with luminescent molybdenum disulfide nanosheet-based peroxidase mimetics

In the present study, we demonstrated that the luminescent molybdenum disulfide (MoS2) nanosheets, which were prepared hydrothermally by using sodium molybdate and thiourea as precursors, possessed peroxidase-like activity, and could catalyze the oxidation of peroxidase substrate o-phenylenediamine (OPD) in the presence of hydrogen peroxide (H2O2) to produce a yellow color reaction. Further addition of Fe2+ into the nanosheets led to peroxidase mimetics with greatly enhanced catalytic activity. The observation was exploited to develop a label-free colorimetric nanozyme sensor for detection of Fe2+. The fabricated MoS2/OPD/H2O2 sensor showed a wide linear range of 0.01–0.8µM with a detection limit of 7nM. Moreover, it was found that the MoS2/OPD/H2O2 sensor displayed enhanced sensitivity and selectivity toward Fe2+ compared with the OPD/H2O2 sensor, suggesting that the MoS2 nanosheets could improve the performance of the Fe2+ sensor. An advanced chemometrics algorithm, multivariate curve resolution by alternating least squares (MCR-ALS), was further applied to interpret the origin of enhancing sensitivity and selectivity in the Fe2+ sensor with the MoS2 nanosheets. The time-dependent UV–vis spectral data of the studied systems were collected, and submitted to the MCR-ALS. The results showed that the increased sensitivity and selectivity of the MoS2/OPD/H2O2 sensor for Fe2+ detection likely arose from its large reaction rate constant. Finally, the proposed MoS2/OPD/H2O2 sensor was successfully applied for detection of Fe2+ in water samples.

A visible and label-free colorimetric sensor for miRNA-21 detection based on peroxidase-like activity of graphene/gold-nanoparticle hybrids

A visible label-free colorimetric sensor for microRNA-21 detection was developed based on the peroxidase-like activity of graphene/gold-nanoparticle hybrids which was flexibly controlled by using single-stranded PNA-21.

A facile label-free colorimetric sensor for Hg2+ based on Hg-triangular silver nanoplates with amalgam-like structure

In this contribution, we have developed a rapid method for the highly selective detection of mercury ions (Hg2+) using citrate-stabilized triangular silver nanoplates (AgNPs). The colorimetric sensor strategy is based on the aggregation of Hg2+-coated AgNPs with the assistance of two amino groups of lysine, resulting in a color change of AgNP solution from blue to mauve or gray. The sensor enables the analysis of Hg2+ with a minimum detectable concentration that corresponds to 50nM. Meanwhile, the strategy offers excellent selectivity toward Hg2+ against other cations. The validation of this method is carried out by analysis of Hg2+ in tap water with satisfying recoveries over 91.35–106%.

A label-free colorimetric sensor for sulfate based on the inhibition of peroxidase-like activity of cysteamine-modified gold nanoparticles

A simple label-free colorimetric sensor for sulfate has been devised based on the analyte-induced shielding of peroxidase-like activity of cysteamine-capped gold nanoparticles (cyst-AuNPs). Aggregation triggered by sulfate through hydrogen bond and electrostatic interaction gave rise to a remarkable decrease of the catalytic activity of AuNPs. By coupling with 3,3′,5,5′-tetramethylbenzidine (TMB)–H2O2 chromogenic reaction, a novel colorimetric sensor for detection of sulfate was established. Based on the signal amplification effect of the catalytic reaction, a detection limit as low as 0.16μM was obtained by virtue of UV–vis spectroscopy or 4μM with the naked-eye observation. The as-proposed sensor exhibited an excellent selectivity over other common anions and metallic ions. In view of these characteristics, such sensing probe features simplicity, cost-effectiveness, and naked-eye screening. These advantages make this sensor a powerful protocol for the quantitative detection of sulfate in water samples with satisfactory results.

Label-free colorimetric sensor for mercury(II) and DNA on the basis of mercury(II) switched-on the oxidase-mimicking activity of silver nanoclusters

In this paper, a novel colorimetric biosensor for Hg2+ and DNA molecules is presented based on Hg2+ stimulated oxidase-like activity of bovine serum albumin protected silver clusters (BSA-Ag NCs). Under mild conditions, Hg2+ activated BSA-Ag NCs to show high catalytic activity toward the oxidation of 3,3′,5, 5′-tetramethylbenzidine (TMB) using ambient dissolved oxygen as an oxidant. The oxidase-like activity of BSA-Ag NCs was “switched-on” selectively in the presence of Hg2+, which permitted a novel and facile colorimetric sensor for Hg2+. As low as 25nmolL−1Hg2+ could be detected with a linear range from 80nmolL−1 to 50mmolL−1. In addition, the sensing strategy was also employed to detect DNA molecules. Hg2+ is known to bind very strongly and specifically with two DNA thymine bases (T) to form thymine–Hg2+–thymine (T–Hg2+–T) base pairs. The hairpin-structure was disrupted and Hg2+ ions were released after hybridization with the DNA target. By coupling the Hg2+ switched-on the oxidase-mimicking activity of BSA-Ag NCs, we developed a novel label-free strategy for facile and fast colorimetric detection of DNA molecules. More important, target DNA can be detected as low as 10nmolL−1 with a linear range from 30 to 225nmolL−1. Compared with other methods, this method presents several advantages such as the independence of hydrogen peroxide, high sensitivity and good selectivity, avoiding any modification or immobilization of DNA, which holds a great potential of metal NCs for clinical application in biosensing and biotechnology.