Masking method for improving selectivity of gold nanoclusters in fluorescence determination of mercury and copperions

Development of an assay for tetracycline detection based on gold nanocluster synthesis on tetracycline monooxygenase: TetX2@AuNCs

Gold nanoclusters were synthesized based on the structure of the TetX2 Monooxygenase enzyme to make a nanocluster based on enzyme structure (TetX2@Au-NCs). Kinetic analysis of TetX2@Au-NC nanozyme revealed that the Km values of TetX2@Au-NCs to both H2O2 and TMB chromogenic substrate components are higher in the absence of tetracycline. Additionally, the Vmax of the nanozyme for TMB increased in the presence of tetracycline. Also, in the presence of H2O2 the Vmax of TetX2@Au-NCs nanozyme for tetracycline was decreased. In order to monitor tetracycline, the peroxidation properties of TetX2@Au-NCs were utilized to change the color of the chromogenic substrate (TMB-H2O2). The catalytic properties of gold nanoclusters were assessed by chemiluminescence reactions, resulting in changes in light emission. Additionally, fluorescence emission changes of TetX2@Au-NCs were monitored at 450 nm. Three different approach including colorimetry, luminescence, and fluorimetry were used to detect tetracycline with detection times and limits of 15 min and 1.3 mM, 1 min and 1.4 mM, and 30 min and 1.6 mM, respectively. Considering changes in temperature, pH, and even high concentrations of the substrate effects on the performance of enzymes, utilizing the TetX2@Au-NCs nanozyme based on TetX2 monooxygenase enzyme proves to be efficient for detecting higher concentrations with increased accuracy and sensitivity.

Dual-emissive phenylalanine dehydrogenase-templated gold nanoclusters as a new highly sensitive label-free ratiometric fluorescent probe: heavy metal ions and thiols measurement with live-cell imaging

Phenylalanine dehydrogenase (PheDH) has been proposed as an ideal protein scaffold for the one-step and green synthesis of highly efficient multifunctional gold nanoclusters. The PheDH-stabilized fluorescent gold nanoclusters (PheDH-AuNCs) with dual emission/single excitation exhibited excellent and long-term stability, high water solubility, large Stokes shift and intense photoluminescence. Selectivity studies demonstrated that the red fluorescence emission intensity of PheDH-AuNCs was obviously decreased in less than 10 min by the addition of mercury, copper, cysteine or glutathione under the single excitation at 360 nm, without significant change in the blue emission of the PheDH-AuNCs. Therefore, the as-prepared PheDH-AuNCs as a new excellent fluorescent probe were successfully employed to develop a simple, rapid, low cost, label- and surface modification-free nanoplatform for the ultrasensitive and selective detection of Hg2+, Cu2+, Cys and GSH through a ratiometric fluorescence system with wide linear ranges and detection limits of 1.6, 2.4, 160 and 350 nM, respectively which were lower than previous reports. In addition, the results showed that PheDH-AuNCs can be used for the detection of toxic heavy metal ions and small biomarker thiols in biological and aqueous samples with acceptable recoveries. Interestingly, PheDH-AuNCs also displayed a promising potential for live-cell imaging due to their low toxicity and great chemical- and photo-stability.

Recent advances in fluorescent materials for mercury(ii) ion detection

Invading mercury would cause many serious health hazards such as kidney damage, genetic freak, and nerve injury to human body. Thus, developing highly efficient and convenient mercury detection methods is of great significance for environmental governance and protection of public health. Motivated by this problem, various testing technologies for detecting trace mercury in the environment, food, medicines or daily chemicals have been developed. Among them, the fluorescence sensing technology is a sensitive and efficient detection method for detecting Hg2+ ions due to its simple operation, rapid response and economic value. This review aims to discuss the recent advances in fluorescent materials for Hg2+ ion detection. We reviewed the Hg2+ sensing materials and divided them into seven categories according to the sensing mechanism: static quenching, photoinduced electron transfer, intramolecular charge transfer, aggregation-induced emission, metallophilic interaction, mercury-induced reactions and ligand-to-metal energy transfer. The challenges and prospects of fluorescent Hg2+ ion probes are briefly presented. We hope that this review can provide some new insights and guidance for the design and development of novel fluorescent Hg2+ ion probes to promote their applications.

A novel ratiometric fluorescence probe for the detection of copper (II) and silver(I) based on assembling dye-doped silica core-shell nanoparticles with gold nanoclusters

A creatively designed and constructed a multifunctional ratiometric fluorescence probe is reported by assembling glutathione (GSH)-protected gold nanoclusters (AuNCs) with fluorescein-doped mesoporous silica nanoparticle (FS) for the detection of Cu²⁺ and Ag⁺ ions, which could eliminate most interferences by self-calibration. Under the excitation at 450 nm, the fluorescence connected with AuNCs can rapidly respond by quenching or enhancement, respectively, for Cu²⁺ and Ag⁺ ions, while the fluorescein isothiocyante (FITC) fluorescence served as reference with negligible change. The fluorescence intensity ratio showed good linear relationships with Cu²⁺ and Ag⁺ concentrations in the range 0.5-10 μM and 0.1-8 μM, respectively. The detection limits were as low as 140 nM and 60 nM for Cu²⁺ and Ag⁺ ions, respectively. The color change induced by fluorescent intensity ratio variation could also be employed for visual discrimination. The AuNC-embedded FS (FS-Au) nanoprobe was successfully used for Cu²⁺ and Ag⁺ ion determination in drinking water and intracellular Cu²⁺ imaging, which exhibits promising prospects in cost-effective and rapid determination of both Cu²⁺ and Ag⁺ with good sensitivity and selectivity.

Photoluminescent Gold/BSA Nanoclusters (AuNC@BSA) as Sensors for Red-Fluorescence Detection of Mycotoxins

The BSA-encapsulated gold nanoclusters (AuNC@BSA) have drawn considerable interest and demonstrated applications as biological sensors. In this study, we demonstrated that the red-emitting AuNC@BSA prepared using a modified procedure fully retained the binding of standard BSA-ligands (small molecule drugs), significantly improving fluorescence detection in some cases due to the red-emission property. Further, we showed that AuNC@BSA efficiently bind a series of aflatoxin-related mycotoxins as well as the aliphatic mycotoxin FB1, reporting interactions in the nanomolar range by instantaneous emission change at 680 nm. Such red emission detection is advantageous over current detection strategies for the same mycotoxins, based on complex mass spectrometry procedures or, eventually (upon chemical modification of the mycotoxin), by fluorescence detection in the UV range (<400 nm). The later technique yields fluorescence strongly overlapping with the intrinsic absorption and emission of biorelevant mixtures in which mycotoxins appear. Thus, here we present a new approach using the AuNC@BSA red fluorescence reporter for mycotoxins as a fast, cheap, and simple detection technique that offers significant advantages over currently available methods.

Constructing a Facile Biocomputing Platform Based on Smart Supramolecular Hydrogel Film Electrodes with Immobilized Enzymes and Gold Nanoclusters

Herein, fluorescent gold nanoclusters (AuNCs) and horseradish peroxidase (HRP) were simultaneously embedded into self-assembled dipeptide supramolecular films of N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) on the surface of ITO electrodes (Fmoc-FF/AuNCs/HRP) by using a simple single-step process. In the films, both the fluorescence property of AuNCs and the bioelectrocatalytic property of HRP were well maintained and could be reversibly regulated by pH-sensitive structural changes in the Fmoc-FF hydrogel films. Cu(II)/EDTA in the solution could lead to the aggregation/disaggregation of AuNCs and further quenching/dequenching the fluorescence signal from the films. Meanwhile, the blue complexes formed by Cu(II) and EDTA could produce a UV-vis signal in the solution. In addition, the coordinated Cu(II) in the films enhanced the electrocatalytic capacity toward the reduction of H₂O₂ and could switch the current signal. A biomolecular logic circuit was built based on the smart film electrode system by using pH, the concentrations of EDTA, Cu(II) and H₂O₂ as inputs, while the fluorescence intensity (FL), current (I) and UV-vis extinction (E) of the solution as outputs. Various logic devices were fabricated using the uniform platform, consisting of an encoder/decoder, demultiplexer, dual-transfer gate, keypad lock, digital comparator, half adder, and controlled NOT (CNOT) gate. Specifically, an electronic three-value logic gate, gullibility (ANY) gate, was first mimicked in this biocomputing system. This work not only demonstrated the construction of a new type of multivalued logic gate by using a dipeptide micromolecular matrix but also provided a new approach for designing sophisticated biologic functions, establishing smart multianalyte biosensing or fabricating biology information processing through the use of a simple film system.

Metal Nanostructures for Environmental Pollutant Detection Based on Fluorescence

Heavy metal ions and pesticides are extremely dangerous for human health and environment and an accurate detection is an essential step to monitor their levels in water. The standard and most used methods for detecting these pollutants are sophisticated and expensive analytical techniques. However, recent technological advancements have allowed the development of alternative techniques based on optical properties of noble metal nanomaterials, which provide many advantages such as ultrasensitive detection, fast turnover, simple protocols, in situ sampling, on-site capability and reduced cost. This paper provides a review of the most common photo-physical effects impact on the fluorescence of metal nanomaterials and how these processes can be exploited for the detection of pollutant species. The final aim is to provide readers with an updated guide on fluorescent metallic nano-systems used as optical sensors of heavy metal ions and pesticides in water.

A novel selective and sensitive multinanozyme colorimetric method for glutathione detection by using an indamine polymer

A sensitive and selective novel multinanozyme colorimetric method for glutathione (GSH) detection was developed. MnO₂-nanozymes can catalyze the oxidation reaction of 3, 3՛-diaminobenzidine (DAB) and produce a brown indamine polymer. In the presence of GSH, this reaction slowly proceeds. When Au-nanozymes was used as peroxidase mimic along with MnO₂-nanozymes, the analytical signal and selectivity (particularly, over Cys and AA) were significantly improved for GSH detection. Therefore, this novel multinanozyme system was further developed through optimization for the colorimetric detection of GSH. The calibration curve presented two wide linear range from 0.05 to 0.19 and 0.19-11.35 mg L^{̶ 1} with a very low detection limit of 0.02 mg L^{̶ 1} (5 nM) for GSH. The developed method was employed for human serum analysis without any dilution and any deproteinization.

Photonic crystal enhanced gold-silver nanoclusters fluorescent sensor for Hg2+ ion

Luminescent nanoclusters (NCs) have attracted much attention because of their good photostability and low toxicity, however, the low quantum yield is still a deficiency, and many increasing efforts are being devoted to enhance the luminescence intensity of NCs. In this paper, a method of enhancing the fluorescent signal of gold-silver nanoclusters (AuAgNCs) by photonic crystals (PhCs) was proposed. The fluorescent intensity of AuAgNCs on PhCs can be enhanced 8.0-fold in comparison to the control sample without PhCs. Furthermore, a novel fluorescence sensor of AuAgNCs based on PhCs is used for the sensitive and selective detection of Hg²⁺ ion in the aqueous solution, the detection limit is 0.35 nM due to the PhCs enhancement effect for the fluorescence. This proposed method may not only develop a highly sensitive method for determination of Hg²⁺ ion, but also expand the application of AuAgNCs in ultra-trace analysis.

Sensitive detection of molybdenum through its catalysis and quenching of gold nanocluster fluorescence

Molybdenum (Mo) is an essential nutrient for the proper functioning of some enzymes in living organisms as human beings. Conventional methods for its detection require complicated instrumentations as atomic absorption or mass spectrometers. In this work, a sensitive kinetic fluorescence was developed as an alternative. Gold nanoclusters (AuNCs) with red fluorescence emission were synthesized, and this fluorescence was effectively quenched by iodine through an etching process. It was found that the presence of Mo significantly speeded up a reaction for the generation of iodine, and thus enhanced the quenching. This effect was then adopted for the development of a sensitive fluorescent measurement toward Mo. The method was capable of detecting Mo down to 0.2 nM and was successfully applied for the analyses of mung bean and tea leaf samples.

Highly selective visual sensing of copper based on fluorescence enhanced glutathione-Au nanoclusters

A blue emission glutathione stabilized Au nanoclusters prepared by an Au/Histidine complex with ligand-exchanges method was used for sensing of copper ions. We found that the glutathione stabilized Au NCs which has fluorescence emission hundred times higher than the Au/Histidine complex and has a highly selective fluorescence quenching response to copper ion. Other common metal ions, such as mercury, lead, iron and zinc, which could obviously quench or enhance the fluorescence of Au/Histidine complex, do not interfere the sensing of copper using glutathione stabilized Au nanocluster. The possible quenching mechanism and the dynamic quenching process for copper detection were also discussed. The results indicated that copper in the range from 0.5 to 300.0μM could be linearly detected and the detection could be finished quickly in 5min. A visual detection method for copper ion that may be used to fast warn copper pollution in waters by naked eyes observation was also be developed using the glutathione stabilized Au NCs probe.

Fluorometric determination of nitrite through its catalytic effect on the oxidation of iodide and subsequent etching of gold nanoclusters by free iodine

A method for sensitive detection of nitrite is presented. It is found that the red fluorescence of gold nanoclusters (with excitation/emission maxima at 365/635 nm) is quenched by traces of iodine via etching. Free iodide is formed by oxidation of iodide by bromate anion under the catalytic effect of nitrite. This catalytic process provides a sensitive means for nitrite detection. Under the optimal conditions, fluorescence linearly dropos in the 10 nM to 0.8 μM nitrite concentration range. The limit of detection is 1.1 nM. This is a few orders of magnitude lower than the maximum concentration allowed by authorities. Graphical abstract Schematic representation of a method for detection of nitrite via a redox reaction. Iodine was produced in the reaction and subsequently quenched the fluorescence from gold nanoclusters by etching their metallic cores, and a sensitive assay for nitrite down to 1.1 nM was developed.

Ratiometric fluorometric determination of mercury(II) by exploiting its quenching effect on glutathione-stabilized and tetraphenylporphyrin modified gold nanoclusters

A ratiometric fluorometric assay for mercury(II) ion is described. It is making use of glutathione-stabilized gold nanoclusters (GSH-AuNCs) modified with tetraphenylporphyrin tetrasulfonic acid (TPPS). The resultant GSH-AuNC/TPPS nanocomposite displays dual emission (at 572 and 664 nm) under a single excitation wavelength of 365 nm. Mercury(II) ion intensively quenches the yellow fluorescence of GSH-AuNCs (peaking at 572 nm) but has a negligible effect on the red fluorescence of TPPS (at 664 nm). The ratio of fluorescence intensities at 572 and 664 nm drops linearly with Hg(II) ion concentration in the 0.02-2.0 μmol·L^-1 range, and the detection limit is 7 nmol·L^-1 (3s_b/S). The relative standard deviation (RSD) of the assay is 2.0% at a 0.5 μmol·L^-1 concentration level (n = 11). The method was successfully applied to the determination of Hg(II) ion in spiked water samples, with recoveries within the range of 87.5-107.5%. Graphical abstract Ratiometric fluorescence detection of mercury(II).

Esterase-Mediated Highly Fluorescent Gold Nanoclusters and Their Use in Ultrasensitive Detection of Mercury: Synthetic and Mechanistic Aspects

The fast, accurate, and ultrasensitive detection of toxic mercury in real water samples is still challenging without the use of expensive sophisticated instruments. Herein, highly fluorescent gold nanoclusters (AuNCs) were synthesized using a newer protein templet, esterase (EST). The EST-AuNCs consisted of ∼25 Au atoms in the nanocluster having ∼2 nm size. EST-AuNCs were found to be highly stable in aqueous buffer with a wide range of pH (pH 4-12) and were also stable in powdered form. The fluorescence quantum yield of EST-AuNCs in deionized water was 6.2% which had increased to 7.8% upon the addition of 1 M NaCl (an increase of 23%). The EST-AuNCs selectively sense the toxic Hg2+ ions with higher sensitivity (limit of detection; 0.88 nM) with the linear range 1-30 nM. The test strips for rapid sensing of Hg2+ in real water samples were developed on the polymeric surface. The validation of sensing ability of EST-AuNCs suggested 94-98% recovery with linearity. Moreover, because of the widely reported applications of EST, the developed EST-AuNCs could also be used for another sensing, catalytic, and biomedical applications.

Chicken Egg White-stabilized Au Nanoclusters for Selective and Sensitive Detection of Hg(II)

In this paper, chicken egg white purchased from a local market without further purification was directly used to prepare fluorescent gold nanoclusters through a one-step, simple, fast and green synthesis approach for analytical purposes. The as-prepared chicken egg white stabilized gold nanocluster probe has strong red fluorescence emission, which can be quenched by mercury ions and copper ions sensitively. By using an ethylenediaminetetraacetate (EDTA) masking method, mercury ions in the range from 0.60 to 10 μM can be linearly detected with the limit of detection (LOD, 3σ) of 0.510 μM in the presence of equivalent copper ions. Since the preparation of a chicken egg white stabilized gold nanocluster probe is fast, easy and cheap, this selective analytical method for mercury pollution monitoring in environmental waters may be widely used in daily life by ordinary people.

Gold nanocluster-based ratiometric fluorescent probes for hydrogen peroxide and enzymatic sensing of uric acid

A method is described for ratiometric fluorometric assays of H₂O₂ by using two probes that have distinct response profiles. Under the catalytic action of ferrous ion, the 615 nm emission of protein-stabilized gold nanoclusters (under 365 nm photoexcitation) is quenched by H₂O₂, while an increased signal is generated with a peak at 450 nm by oxidizing coumarin with the H₂O₂/Fe(II) system to form a blue emitting fluorophore. These decrease/increase responses give a ratiometric signal. The ratio of the fluorescences at the two peaks are linearly related to the concentration of H₂O₂ in the range from 0.05 to 10 μM, with a 7.7 nM limit of detection. The detection scheme was further coupled to the urate oxidase catalyzed oxidation of uric acid which proceeds under the formation of H₂O₂. This method provides an simple and effective means for the construction of ratiometric fluorometric (enzymatic) assays that involve the detection of H₂O₂. Graphical abstract Under catalysis by ferrous ion, hydrogen peroxide quenches the luminescence of gold nanoclusters (AuNCs) and oxidizes coumarin into a fluorescent derivative, which rendered fluorescence ON and OFF at two distinct wavelengths for ratiometric measurements.

Copper ion detection with improved sensitivity through catalytic quenching of gold nanocluster fluorescence

In this report, a sensitive fluorescence detection of copper (Ⅱ) ion was developed. Although itself only a weak quencher toward gold nanocluster fluorescence, this ion functioned as a catalyst that accelerated the oxidation of iodide into iodine by a strong oxidant. The so-produced iodine quenched the nanocluster fluorescence through an efficient etching reaction, which rendered a much improved sensitivity for copper detection. Under the optimal conditions, the extent of quenching was found linear to the amount of copper in the range of 0.8-80 nM, and this strategy was capable of detecting copper ion as low as 0.33 nM. The method was selective and was successfully applied for related measurement in practical samples.

Mercaptosuccinic acid-coated NIR-emitting gold nanoparticles for the sensitive and selective detection of Hg2

A sensitive fluorescent detection platform for Hg²⁺ was constructed based on mercaptosuccinic acid (MSA) coated near-infrared (NIR)-emitting gold nanoparticles (AuNPs). The thiolated mercaptosuccinic acid was employed as both reducing agent and surface coating ligand in a one-step synthesis of NIR-emitting AuNPs (MSA-AuNPs), which exhibited stable fluorescence with the maximum wavelength at 800nm and a wide range of excitation (220-650nm) with the maxima at 413nm. The MSA coated NIR-emitting AuNPs showed a rapid fluorescence quenching toward Hg²⁺ over other metal ions with a limit of detection (LOD, 3δ) as low as 4.8nM. The sensing mechanism investigation revealed that the AuNPs formed aggregation due to the "recognition" of Hg²⁺ from the MSA, and the resultant strong coupling interaction between Hg²⁺ and Au (I) to further quench the fluorescence of the AuNPs, which synergistically resulted in a highly sensitive and selective fluorescence response toward Hg²⁺. This proposed strategy was also demonstrated the possibility to be used for Hg²⁺ detection in water samples.

Cytidine-stabilized copper nanoclusters as a fluorescent probe for sensing of copper ions and hemin

We reported a sensitive and selective fluorescence “turn on–off” strategy for detection of Cu²⁺ and hemin, respectively. The fluorescence “turn on” sensor for Cu²⁺ detection had a wide linear range of 0.05–2.0 μM with a limit of detection (LOD) of 0.032 μM, and the fluorescence “turn off” sensor for hemin detection possessed a wide linear range of 0.05–4.0 μM with an LOD of 0.045 μM. The sensor for Cu²⁺ or hemin exhibited high selectivity over other possible substances. In addition, it was demonstrated by using various analytical characterization techniques that the fluorescence “turn on” sensor for Cu²⁺ was constructed on the basis of the formation of water-soluble fluorescent copper nanoclusters (CuNCs), and the fabrication of the fluorescence “turn off” sensor for hemin was predominately based on the inner filter effect of hemin on the fluorescence of the CuNCs. Finally, the proposed fluorescence “turn on–off” sensor system was successfully applied for detection of Cu²⁺ in lake water samples and hemin in duck blood samples.

A sensitive and selective fluorescence “turn on–off” strategy for simultaneous detection of Cu²⁺ and hemin was proposed on the basis of the formation of fluorescent CuNCs and the inner filter effect of hemin on the fluorescence of the CuNCs.

Highly fluorescent gold nanoclusters stabilized by food proteins: From preparation to application in detection of food contaminants and bioactive nutrients

Applications of nanotechnology in food have rapidly increased in the past decades. Ultra-small gold nanoclusters (Au NCs), composed of several to roughly a hundred atoms, represent a kind of novel nanomaterials. The Au NCs directed by food proteins have drawn considerable research attention due to their environment-friendly preparation, strong fluorescence, excellent photo-stability, and favorable biocompatibility. These interesting protein-Au hybrids have opened up a new area at the nano-bio-food interface, not only did they provide the missing link between single metal atoms and plasmonic metal nanoparticles, but also developed the hybrid system between biomacromolecule and inorganic ions. In this review, we highlighted the synthesis strategies and optical properties of the Au NCs stabilized by typical food proteins as well as their applications in detection of food contaminants or bioactive nutrients. In addition, we discussed current challenges and future development in food proteins- directed gold nanoclusters for size-controlled synthesis and multifunctional applications.

A “turn-off” fluorescent sensor for the selective and sensitive detection of copper(ii) ions using lysozyme stabilized gold nanoclusters

In this contribution, we have developed a simple, environmentally friendly fluorescent turn-off sensor for the detection of copper (Cu²⁺) ions in aqueous solution by using lysozyme stabilized gold nanoclusters (Lys-AuNCs) as a fluorescent probe.

Nitrite ion-induced fluorescence quenching of luminescent BSA-Au(25) nanoclusters: mechanism and application

Fluorescence quenching is an interesting phenomenon which is highly useful in developing fluorescence based sensors. A thorough understanding of the fluorescence quenching mechanism is essential to develop efficient sensors. In this work, we investigate different aspects governing the nitrite ion-induced fluorescence quenching of luminescent bovine serum albumin stabilized gold nanoclusters (BSA-Au NCs) and their application for detection of nitrite in urine. The probable events leading to photoluminescence (PL) quenching by nitrite ions were discussed on the basis of the results obtained from ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), fluorescence measurements, circular dichroism (CD) spectroscopy, zeta potential and dynamic light scattering (DLS) studies. These studies suggested that PL quenching mainly occurred through the oxidation of Au(0) atoms to Au(i) atoms in the core of BSA-Au NCs mediated by nitrite ions. The interference caused by certain species such as Hg(2+), Cu(2+), CN(-), S(2-), glutathione, cysteine, etc. during the nitrite determination by fluorescence quenching was eliminated by using masking agents and optimising the conditions. Based on these findings we proposed a BSA-Au NC-modified membrane based sensor which would be more convenient for the real life applications such as nitrite detection in urine samples. The BSA-Au NC-modified nitrocellulose membrane (NCM) enabled the detection of nitrite at a level as low as 100 nM in aqueous solutions. This Au NC-based paper probe was validated to exhibit good performance for nitrite analysis in environmental water and urine samples, which makes it useful in practical applications.

Silver Nanoparticle-Enhanced Resonance Raman Sensor of Chromium(III) in Seawater Samples

Tris(hydroxymethyl)aminomethane ethylenediaminetetraacetic acid (Tris-EDTA), upon binding Cr(III) in aqueous solutions at pH 8.0 on silver nanoparticles (AgNPs), was found to provide a sensitive and selective Raman marker band at ~563 cm-1, which can be ascribed to the metal-N band. UV-Vis absorption spectra also supported the aggregation and structural change of EDTA upon binding Cr(III). Only for Cr(III) concentrations above 500 nM, the band at ~563 cm-1 become strongly intensified in the surface-enhanced Raman scattering spectra. This band, due to the metal-EDTA complex, was not observed in the case of 50 mM of K+, Cd2+, Mg2+, Ca2+, Mn2+, Co2+, Na+, Cu2+, NH4+, Hg2+, Ni2+, Fe3+, Pb2+, Fe2+, and Zn2+ ions. Seawater samples containing K, Mg, Ca, and Na ion concentrations higher than 8 mM also showed the characteristic Raman band at ~563 cm-1 above 500 nM, validating our method. Our approach may be useful in detecting real water samples by means of AgNPs and Raman spectroscopy.

Selection of a DNA aptamer for cadmium detection based on cationic polymer mediated aggregation of gold nanoparticles

The demand for selection of aptamers against various small chemical molecules has substantially increased in recent years. To incubate and separate target-specific aptamers, the conventional SELEX procedures generally need to immobilize target molecules on a matrix, which may be impotent to screen aptamers toward small molecules without enough sites for immobilization. Herein we chose Cd(II) as a model of a small molecule with less sites, and proposed a novel SELEX strategy of immobilizing ssDNA libraries rather than target molecules on a matrix, for selection of aptamers with high affinity to Cd(II). After eleven rounds of positive and negative selection, twelve T and G-rich of nonrepeating ssDNA sequences were identified, of which the Cd-4 aptamer displayed the highest binding affinity to Cd(II). The secondary structures of these sequences revealed that a stem-loop structure folded by the domain of their 30-random sequence is critical for aptamers to bind targets. Then the interaction between the selected Cd-4 aptamer and Cd(II) was confirmed by CD analysis, and the binding specificity toward other competitive metal ions was also investigated. The dissociation constant (Kd) of Cd-4 aptamer was determined as 34.5 nM for Cd(II). Moreover, the Cd-4 aptamer was considered a recognition element for the colorimetric detection of Cd(II) based on the aggregation of AuNPs by cationic polymer. Through spectroscopic quantitative analysis, Cd(II) in aqueous solution can be detected as low as 4.6 nM. The selected Cd-4 aptamer will offer a new substitute for the detection of Cd(II) or other applications like recovery of cadmium from polluted samples.

Sensitive detection of mercury and copper ions by fluorescent DNA/Ag nanoclusters in guanine-rich DNA hybridization

In this work, we designed a new fluorescent oligonucleotides-stabilized silver nanoclusters (DNA/AgNCs) probe for sensitive detection of mercury and copper ions. This probe contains two tailored DNA sequence. One is a signal probe contains a cytosine-rich sequence template for AgNCs synthesis and link sequence at both ends. The other is a guanine-rich sequence for signal enhancement and link sequence complementary to the link sequence of the signal probe. After hybridization, the fluorescence of hybridized double-strand DNA/AgNCs is 200-fold enhanced based on the fluorescence enhancement effect of DNA/AgNCs in proximity of guanine-rich DNA sequence. The double-strand DNA/AgNCs probe is brighter and stable than that of single-strand DNA/AgNCs, and more importantly, can be used as novel fluorescent probes for detecting mercury and copper ions. Mercury and copper ions in the range of 6.0-160.0 and 6-240 nM, can be linearly detected with the detection limits of 2.1 and 3.4 nM, respectively. Our results indicated that the analytical parameters of the method for mercury and copper ions detection are much better than which using a single-strand DNA/AgNCs.

Furthering the chemosensing of silver nanoclusters for ion detection

An eco-friendly silver nanocluster chemosensor for Mn²⁺and I⁻ion detection, differentiation and bioimaging was synthesized. The chemosensing mechanisms were elucidated by microscopic characterization and spectral analyses.

A novel biosensor for copper(ii) ions based on turn-on resonance light scattering of ssDNA templated silver nanoclusters

A new ultrasensitive biosensor for copper(ii) ions was first developed based on turn-on resonance light scattering (RLS) of ssDNA templated silver nanoclusters through anti-galvanic reduction (AGR).

Hydrogen peroxide sensitive hemoglobin-capped gold nanoclusters as a fluorescence enhancing sensor for the label-free detection of glucose

A novel label-free fluorescent probe based on blue-emitting gold nanoclusters capped by hemoglobin for the direct detection of glucose is presented.

Interactions of nitroxide radicals with dendrimer-entrapped Au8-clusters: a fluorescent nanosensor for intracellular imaging of ascorbic acid

When gold nanoparticles (AuNPs) become extremely small (<2 nm in diameter) as gold nanoclusters (AuNCs), an intriguing issue is whether the interactions of free radicals with AuNCs would be essentially different at sufficiently small size. Herein, we report for the first time that the fluorescence of a polyamidoamine (PAMAM) dendrimer-entrapped Au₈-cluster is quenched by the paramagnetic nitroxide radical. Based on an upward curving Stern-Volmer plot, the system shows complex fluorescence quenching with a combination of static and dynamic quenching processes. The quenching mechanism associated with the interactions between Au₈-clusters and nitroxide radicals was explored by combined fluorescence and electron paramagnetic resonance (EPR) studies. The controlled quenching of the fluorescent Au₈-cluster can be developed as a turn-on fluorescence probe for sensing ascorbic acid (AA) in living cells.

A label-free fluorescent assay for free chlorine in drinking water based on protein-stabilized gold nanoclusters

Bovine serum albumin stabilized Au nanoclusters (BSA-AuNCs) were demonstrated as a novel fluorescence probe for sensitive and selective detection of free chlorine in drinking water. The fluorescence of BSA-AuNCs was found to be quenched effectively by the free chlorine, and the decrease in fluorescence intensity of BSA-AuNCs allowed the sensitive detection of free chlorine in the range of 0.8-800 μM. The detection limit is 0.50 μM at a signal-to-noise ratio of 3. The present fluorescent assay for free chlorine possesses low detection limit, wide linear range and good selectivity. Real tap water samples were analyzed with satisfactory results, which suggested its potential for water quality analysis.

Self-assembly of hybridized ligands on gold nanodots: tunable photoluminescence and sensing of nitrite

Highly photoluminescent gold nanodots (Au NDs) via etching and co-deposition of hybridized ligands [11-mercaptoundecanol (11-MU) and its complexes with amphiphilic ligands] on gold nanoparticles (∼3 nm) have been prepared and employed for the detection of nitrite based on the analyte-induced photoluminescence quenching.