Colorimetric assay of lead ions in biological samples using a nanogold-based membrane

Pb2+ Ion Sensors Employing Gold Etching Process: Comparative Investigation on Au Nanorods and Au Nanotriangles

The leaching phenomenon of gold (Au) nanomaterials by Pb2+ ions in the presence of 2-mercaptoethanol (2-ME) and thiosulfate (S2O32- ion) has been systematically applied to a Pb2+ ion sensor. To further investigate the role of Pb2+ ions in sensors containing Au nanomaterials, we revisited the leaching conditions for Au nanorods and compared them with the results for Au nanotriangles. By monitoring the etching rate, it was revealed that Pb2+ ions were important for the acceleration of the etching rate mainly driven by 2-ME and S2O32- pairs, and nanomolar detection of Pb2+ ions were shown to be promoted through this catalytic effect. Using the etchant, the overall size of the Au nanorods decreased but showed an unusual red-shift in UV-Vis spectrum indicating increase of aspect ratio. Indeed, the length of Au nanorods decreased by 9.4% with the width decreasing by 17.4% over a 30-min reaction time. On the other hand, the Au nanotriangles with both flat sides surrounded mostly by dense Au{111} planes showed ordinary blue-shift in UV-Vis spectrum as the length of one side was reduced by 21.3%. By observing the changes in the two types of Au nanomaterials, we inferred that there was facet-dependent alloy formation with lead, and this difference resulted in Au nanotriangles showing good sensitivity, but lower detection limits compared to the Au nanorods.

Valorised polypropylene waste based reversible sensor for copper ion detection in blood and water

Heavy metals and plastic pollutants are the two most disastrous challenges to the environment requiring immediate actions. In this work, a techno-commercially feasible approach to address both challenges is presented, where a waste polypropylene (PP) based reversible sensor is produced to selectively detect copper ions (Cu2+) in blood and water from different sources. The waste PP-based sensor was fabricated in the form of an emulsion-templated porous scaffold decorated with benzothiazolinium spiropyran (BTS), which produced a reddish colour upon exposure to Cu2+. The presence of Cu2+ was checked by naked eye, UV-Vis spectroscopy, and DC (Direct Current) probe station by measuring the current where the sensor's performance remained unaffected while analysing blood, water from different sources, and acidic or basic environment. The sensor exhibited 1.3 ppm as the limit of detection value in agreement with the WHO recommendations. The reversible nature of the sensor was determined by cyclic exposure of the sensor towards visible light turning it from coloured to colourless within 5 min and regenerated the sensor for the subsequent analysis. The reversibility of the sensor through exchange between Cu2+- Cu+ was confirmed by XPS analysis. A resettable and multi-readout INHIBIT logic gate was proposed for the sensor using Cu2+ and visible light as the inputs and colour change, reflectance band and current as the output. The cost-effective sensor enabled rapid detection of the presence of Cu2+ in both water and complex biological samples such as blood. While the approach developed in this study provides a unique opportunity to address the environmental burden of plastic waste management, it also allows for the possible valorization of plastics for use in enormous value-added applications.

Monitoring Cd2+ in oily wastewater using an aptamer-graphene field-effect transistor with a selective wetting surface

The discharge of oily industrial wastewater containing heavy metal ions with the development of industry severely threatens the environment and human health. Therefore, it is of great significance to monitor the concentration of heavy metal ions in oily wastewater quickly and effectively. Here, an integrated Cd²⁺ monitoring system consisting of an aptamer-graphene field-effect transistor (A-GFET), oleophobic/hydrophilic surface and monitoring-alarm circuits was presented for monitoring the Cd²⁺ concentration in oily wastewater. In the system, oil and other impurities in wastewater are isolated by an oleophobic/hydrophilic membrane before detection. The concentration of Cd²⁺ is then detected by a graphene field-effect transistor with a Cd²⁺ aptamer modifying the graphene channel. Finally, the detected signal is collected and processed by signal processing circuits to judge whether the Cd²⁺ concentration exceeds the standard. Experimental results demonstrated that the separation efficiency of the oleophobic/hydrophilic membrane to an oil/water mixture was up to 99.9%, exhibiting a high oil/water separation ability. The A-GFET detecting platform could respond to changes in the Cd²⁺ concentration within 10 min with a limit of detection (LOD) of 0.125 pM. The sensitivity of this detection platform to Cd²⁺ near 1 nM was 7.643 × 10^-2 nM^-1. Compared with control ions (Cr³⁺, Pb²⁺, Mg²⁺, Fe³⁺), this detection platform exhibited a high specificity to Cd²⁺. Moreover, the system could send out a photoacoustic alarm signal when the Cd²⁺ concentration in the monitoring solution exceeds the preset value. Therefore, the system is practical for monitoring the concentration of heavy metal ions in oily wastewater.

Recent advances in colorimetric sensors based on nanozymes with peroxidase-like activity

Nanozymes have been widely used to construct colorimetric sensors due to their advantages of cost-effectiveness, high stability, good biocompatibility, and ease of modification. The emergence of nanozymes greatly enhanced the detection sensitivity and stability of the colorimetric sensing platform. Recent significant research has focused on designing various sensors based on nanozymes with peroxidase-like activity for colorimetric analysis. However, with the deepening of research, nanozymes with peroxidase-like activity has also exposed some problems, such as weak affinity and low catalytic activity. In view of the above issues, existing investigations have shown that the catalytic properties of nanozymes can be improved by adding surface modification and changing the structure of nanomaterials. In this review, we summarize the recent trends and advances of colorimetric sensors based on several typical nanozymes with peroxidase-like activities, including noble metals, metal oxides, metal sulfides/metal selenides, and carbon and metal-organic frameworks (MOF). Finally, the current challenges and prospects of colorimetric sensors based on nanozymes with peroxidase-like activity are summarized and discussed to provide a reference for researchers in related fields.

Plasmonic Nanomaterials for Colorimetric Biosensing: A Review

In the last few decades, plasmonic colorimetric biosensors raised increasing interest in bioanalytics thanks to their cost-effectiveness, responsiveness, and simplicity as compared to conventional laboratory techniques. Potential high-throughput screening and easy-to-use assay procedures make them also suitable for realizing point of care devices. Nevertheless, several challenges such as fabrication complexity, laborious biofunctionalization, and poor sensitivity compromise their technological transfer from research laboratories to industry and, hence, still hamper their adoption on large-scale. However, newly-developing plasmonic colorimetric biosensors boast impressive sensing performance in terms of sensitivity, dynamic range, limit of detection, reliability, and specificity thereby continuously encouraging further researches. In this review, recently reported plasmonic colorimetric biosensors are discussed with a focus on the following categories: (i) on-platform-based (localized surface plasmon resonance, coupled plasmon resonance and surface lattice resonance); (ii) colloid aggregation-based (label-based and label free); (iii) colloid non-aggregation-based (nanozyme, etching-based and growth-based).

Gold nanostar as an ultrasensitive colorimetric probe for picomolar detection of lead ion

The sensitivity for analytes of interest is vital for environment protection and food safety. Here, we propose an extremely sensitive assay toward Pb²⁺ by using gold nanostars (GNSs) as probes based on the catalytic activity of Pb on etching gold atoms after being reduced in the presence of 2-mercaptoethanol (2-ME) and sodium thiosulfate. GNSs were prepared by using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid as both the reducing and capping agents, enabling high stability and sensitivity for quantitation of Pb²⁺. Upon increasing Pb²⁺ concentration over the range of 0-10 μM, GNS solution color changed from greenish-blue to blue to purple to red, and eventually to colorless. The color change can be distinguished by naked eye at the Pb²⁺ concentration as low as 200 pM. Through monitoring longitudinal localized surface plasmon of GNSs, Pb²⁺ could be detected with a limit of detection of 1.5 pM, and the working range is 2 pM-1 μM. The ultra-high sensitivity of our assay stems from the high catalysis of Pb on etching gold on tips and branches in the presence of 2-ME and sodium thiosulfate, leading to the shape deformation to spherical gold nanoparticle and the corresponding significant changes in their optical properties. The assay provides high selectivity of Pb²⁺ over the tested interfering metal ions like Cu²⁺. With high sensitivity and selectivity, the assay was efficiently validated by analyzing water samples and monitoring the migration of Pb²⁺ from the tested container to water.

A phenanthrene[9,10-d]imidazole-phenol-based fluorescent probe combining ESIPT and AIE for the “turn-on” detection of Cu2+ with green-emission and improved Stokes’ shift, and its application

The “turn-on” probe PIA(OH)-Py responds to Cu2+ in living cells and can determine the concentration of Cu2+ in blood samples.

DNA-Gold Nanozyme-Modified Paper Device for Enhanced Colorimetric Detection of Mercury Ions

In this work, a paper device consisted of a patterned paper chip, wicking pads, and a base was fabricated. On the paper chip, DNA–gold nanoparticles (DNA–AuNPs) were deposited and Hg²⁺ ions could be adsorbed by the DNA–AuNPs. The formed DNA–AuNP/Hg²⁺ nanozyme could catalyze the tetramethylbenzidine (TMB)–H₂O₂ chromogenic reaction. Due to the wicking pads, a larger volume of Hg²⁺ sample could be applied to the paper device for Hg²⁺ detection and therefore the color response could be enhanced. The paper device achieved a cut-off value of 50 nM by the naked eye for Hg²⁺ under optimized conditions. Moreover, quantitative measurements could be implemented by using a desktop scanner and extracting grayscale values. A linear range of 50–2000 nM Hg²⁺ was obtained with a detection limit of 10 nM. In addition, the paper device could be applied in the detection of environmental water samples with high recoveries ranging from 85.7% to 105.6%. The paper-device-based colorimetric detection was low-cost, simple, and demonstrated high potential in real-sample applications.

Analysis of lead forms and transition in agricultural soil by nano-fluorescence method

The physicochemical properties of gold nanorods were used to develop many methods and techniques of detecting heavy metals recently. In this paper, the feasibility of gold nanorods was studied to detect metal lead in agricultural soil. The effects of soil properties on the form change of lead in soil were explored by gold nanorod detection technology. The results showed that the humic acid significantly increased Pb mobility and ion state exchanging. It also increased the lead content of organic bound state and Fe-Mn oxides state. And the detection process by gold nanorods proved to be a more simple and convenient method.

Novel formation of Au/Ag bimetallic nanoparticles from a mixture of monometallic nanoparticles and their application for the rapid detection of lead in onion samples

Characterization of gold, silver and gold/silver bimetallic nanoparticles for colorimetric detection of lead in onion samples.

Gold Nanoparticle-Based Colorimetric Strategies for Chemical and Biological Sensing Applications

Gold nanoparticles are popularly used in biological and chemical sensors and their applications owing to their fascinating chemical, optical, and catalytic properties. Particularly, the use of gold nanoparticles is widespread in colorimetric assays because of their simple, cost-effective fabrication, and ease of use. More importantly, the gold nanoparticle sensor response is a visual change in color, which allows easy interpretation of results. Therefore, many studies of gold nanoparticle-based colorimetric methods have been reported, and some review articles published over the past years. Most reviews focus exclusively on a single gold nanoparticle-based colorimetric technique for one analyte of interest. In this review, we focus on the current developments in different colorimetric assay designs for the sensing of various chemical and biological samples. We summarize and classify the sensing strategies and mechanism analyses of gold nanoparticle-based detection. Additionally, typical examples of recently developed gold nanoparticle-based colorimetric methods and their applications in the detection of various analytes are presented and discussed comprehensively.

Novel double layer film composed of reduced graphene oxide and Rose Bengal dye: design, fabrication and evaluation as an efficient chemosensor for silver(i) detection

A novel silver-chemosensor fabricated with reduced graphene oxide and Rose Bengal (RB) based on the interaction of Ag⁺ and RB.

Asymmetric Nanochannel-Ionchannel Hybrid for Ultrasensitive and Label-Free Detection of Copper Ions in Blood

Nanochannel/nanopre based analysis methods have attracted increasing interest in recent years due to their exquisite ability to reveal changes in molecular volume. In this work, a highly asymmetric nanochannel-ionchannel hybrid coupled with an electrochemical technique was developed for copper ion (Cu²⁺) detection. Polyglutamic acid (PGA) was modified in a nanochannel array of porous anodic alumina (PAA). When different concentrations of Cu²⁺ were introduced into the nanochannel-ionchannel hybrid in a neutral environment, a Cu²⁺-PGA chelation reaction occurs, resulting in varied current-potential (I-V) properties of the nanochannel-ionchannel hybrid. When PAA was immersed in a low pH solution, the Cu²⁺-PGA complex dissociated. On the basis of the change in ionic current, a label-free assay for Cu²⁺ was achieved along with the ability to regenerate and reuse the constructed platform. Because of the unique mass transfer property of the nanochannel-ionchannel hybrid combined with the highly amplified ionic current magnitude of the nanochannel array, significantly increased assay sensitivity was achieved, as expected. To evaluate the applicability of the present methodology for detecting Cu²⁺ in a real sample, the Cu²⁺ content in real blood samples was analyzed. The results demonstrate that the present method shows excellent selectivity with high sensitivity toward Cu²⁺ detection in real blood samples.

Bionanotechnology-Based Colorimetric Sensors for Food Analysis

Colorimetric biosensing is widely used in clinical diagnosis and environmental evaluation due to its simplicity and practicality. It has also recently become popular in food analysis. Nanotechnology is being integrated into the development of colorimetric biosensors to overcome the bottleneck of conventional colorimetric biosensing approaches. Innovative bionanotechnology-based colorimetric sensors have recently been developed. This chapter focuses on the progress of bionanotechnology-based colorimetric biosensors in food safety assessment. We also describe how nanomaterials can be integrated and tailored to meet the requirements of colorimetric biosensing systems for the detection of heavy metal cations, antibiotics, nucleic acids, and toxins/toxicants. Approaches described include functionalization of nanomaterials to act as colorimetric probes, carriers and enzyme mimetics. Selected examples of the most recent preliminary applications of bionanotechnology-based colorimetric biosensors in food safety assessment are given to illustrate the novel concepts and promising future applications. Future prospects for the application of bionanotechnology-based colorimetric biosensors in food safety assessment are also briefly discussed.

Paper-based analytical device for sampling, on-site preconcentration and detection of ppb lead in water

A simple and cost effectiveness procedure based on a paper based analytical device (PAD) for sampling, on-site preconcentration and determination of Pb(II) in water samples was developed. The inkjet printing method was used for patterning of PAD. Colorimetric assay was developed on a PAD for Pb(II) detection in µgL(-1) level. This µgL(-1) level detection limit was achieved by in situ- and on-site preconcentration of Pb(II) onto adsorption filter paper disc with a home-made holder before color development. Water sample was loaded onto a circular filter paper coated with zirconium silicate in 3% sodium carboxymethylcellulose for Pb(II) preconcentration. Subsequently, sodium rhodizonate in tartrate buffer solution (pH 2.8) was used as colorimetric reagent for direct Pb(II) detection on a PAD. Detection was achieved by measuring the pink color and recorded by scanner or digital camera. ImageJ software was used for measuring grey scale values. The calibration curve was linear in the range of 10µgL(-1) and 100µgL(-1), with a detection limit of 10µgL(-1). The developed method was successfully applied to the determination of Pb(II) in drinking water, tap water and surface water near electronic waste storage and the results were compared with those by graphite furnace atomic absorption spectroscopy (GF-AAS) with good agreement.

Ultra-sensitive fluorescence determination of chromium(vi) in aqueous solution based on selectively etching of protein-stabled gold nanoclusters

In this work, we have developed a simple, cost-effective and sensitive fluorescent method for the selective determination of chromium(vi) ions (Cr(vi)) in aqueous solution.

Fenton-like reaction-mediated etching of gold nanorods for visual detection of Co(2+)

We have proposed a Fenton-like reaction-mediated etching of gold nanorods and applied it to the sensitive visual detection of Co(2+) ions. With the presence of bicarbonate (HCO3(-)) and hydrogen peroxide(H2O2), Co(2+) ions trigger a Fenton-like reaction, resulting in the generation of superoxide radical (O2(•-)). As a result, the gold nanorods are gradually etched by O2(•-) in the presence of SCN(-), accompanied by an obvious color change from green to red. The gold nanorods etching process preferentially occurs along the longitudinal direction, which is observed by transmission electron microscope. The etching mechanism is carefully proved by investigating the effects of different radical scavengers (e.g., dimethyl sulfoxide). The auto-oxidation of hydroxylamine assay further confirms the mechanism. Then, the main factors, including reactants concentrations, temperature, and incubation time, are specifically investigated. Under optimized conditions, we get an excellent sensing performance for Co(2+) with a lower detection limit of 1.0 nM via a spectrophotometer and a visual detection limit of 40 nM. In addition, this principle may provide a new concept of "intermediate-mediated etching of nanoparticles" for sensing.

Functionalization of cationic poly(p-phenylene ethynylene) with dendritic polyethylene enables efficient DNAzyme delivery for imaging Pb2+ in living cells

We report here an effective Pb²⁺-dependent DNAzyme (8-17 DNAzyme) delivery system based on the water-soluble dendritic polyethylene-cationic poly(p-phenylene ethynylene) for successfully imaging Pb²⁺ in living cells. For utilizing the 8-17 DNAzyme and its unique ability to catalyze a phosphodiester bond cleavage reaction in the presence of Pb²⁺, the distinctive conjugated polymer-based polyvalent nanocarrier design manages to load and transport 8-17 DNAzyme across cell membranes, and to realize the fluorescence imaging of Pb²⁺ in living cells. As shown by the confocal microscopy and flow cytometry observations, the fluorescence of Cy5.5 is obviously activated under the conditions of incubation with Pb²⁺, compared with the absence of Pb²⁺. Taken together, the study demonstrates the combination of the molecular-wire effect with "dendrimer effects" on their effective DNAzyme delivery and their cellular imaging Pb²⁺.

Sensitive Pb(2+) probe based on the fluorescence quenching by graphene oxide and enhancement of the leaching of gold nanoparticles

A novel strategy was developed for fluorescent detection of Pb(2+) in aqueous solution based on the fact that graphene oxide (GO) could quench the fluorescence of amino pyrene (AP)-grafted gold nanoparticles (AP-AuNPs) and Pb(2+) could accelerate the leaching rate of AuNPs in the presence of S2O3(2-). In this system, fluorescence reporter AP was grafted on AuNPs through the Au-N bond. In the presence of GO, the system shows fluorescence quenching because of π-π stacking between AP and GO. With the addition of Pb(2+) and S2O3(2-), the system displays fluorescence recovery, which is attributed to the fact that Pb(2+) could accelerate the leaching of the AuNPs from GO surfaces and release of AP into aqueous solution. Interestingly, the concentration of GO could control the fluorescence "turn-off" or "turn-on" for Pb(2+) detection. In addition, GO is also an excellent promoter for the acceleration of the leaching of AuNPs and shortening the analytical time to ∼15 min. Under the optimal conditions, the fluorescence Pb(2+) sensor shows a linear range from 2.0 × 10(-9) to 2.3 × 10(-7) mol/L, with a detection limit of 1.0 × 10(-10) mol/L.

Fluorescent detection of lead in environmental water and urine samples using enzyme mimics of catechin-synthesized Au nanoparticles

A facile, cost-effective, and sensitive fluorescent method for Pb²⁺ ion detection had been developed using catechin synthesized gold nanoparticles (C-Au NPs). The Pb-catechin complexes and Pb-Au alloys that formed on the C-Au NPs surfaces allowed NPs to exhibit peroxidase-mimicking catalytic activity in the H₂O₂-mediated oxidation of Amplex UltraRed (AUR). In 5 mM Tris-acetate buffers at pH 7.0, the H₂O₂-AUR-C-Au NP probe was highly selective (>100-fold) for Pb²⁺ ions in the presence of other tested metal ions (K⁺, Ag⁺, Na⁺, Cd²⁺, Ni²⁺, Ca²⁺, Hg²⁺, Sr²⁺, Co²⁺, Cu²⁺, Ba²⁺, Fe²⁺, Mg²⁺, Cr³⁺, and Fe³⁺ ions). The fluorescence intensity (excitation/emission maxima ∼540/588 nm) of the AUR product was proportional to the concentration of Pb²⁺ ions in the range of 10 nM-1.0 μM with a linear correlation (R² = 0.99). The H₂O₂-AUR-C-Au NP probe detected Pb²⁺ ions with a limit of detection (signal-to-noise ratio: 3) of 1.5 nM. The practicality of the H₂O₂-AUR-C-Au NP probe was validated for the determination of Pb²⁺ ion concentration in environmental water and urine samples, demonstrating its advantages of simplicity, selectivity, and sensitivity.

Nanostructured substrates for portable and miniature SPR biosensors

Surface plasmon resonance (SPR) biosensing has matured into a valuable analytical technique for measurements related to biomolecules, environmental contaminants, and the food industry. Contemporary SPR instruments are mainly suitable for laboratory-based measurements. However, several point-of-measurement applications would benefit from simple, small, portable and inexpensive sensors to assess the health condition of a patient, potential environmental contamination, or food safety issues. This Trend article explores nanostructured substrates for improving the sensitivity of classical SPR instruments and nanoparticle (NP)-based colorimetric substrates that may provide a solution to the development of point-of-measurement SPR techniques. Novel nanomaterials and methodology capable of enhancing the sensitivity of classical SPR sensors are destined to improve the limits of detection of miniature SPR instruments to the level required for most applications. In a different approach, paper or substrate-based SPR assays based on NPs, are a highly promising topic of research that may facilitate the widespread use of a novel class of miniature and portable SPR instruments.