Multiplexed analysis of silver(I) and mercury(II) ions using oligonucletide-metal nanoparticle conjugates

A colorimetric assay has been developed for the simultaneous selective detection of silver(I) and mercury(II) ions utilizing metal nanoparticles (NPs) as sensing element based on their unique surface plasmon resonance properties. In this method, sulfhydryl group modified cytosine-(C)-rich ssDNA (SH-C-ssDNA) was self-assembled on gold nanoparticles (AuNPs) to produce the AuNPs-C-ssDNA complex, and sulfhydryl group modified thymine-(T)-rich ssDNA (SH-T-ssDNA) was self-assembled on silver nanoparticles (AgNPs) to produce the AgNPs-T-ssDNA complex. Oligonucleotides (SH-C-ssDNA or SH-T-ssDNA) could enhance the AuNPs or AgNPs against salt-induced aggregation. However, the presence of silver(I) ions (Ag(+)) in the complex of ssDNA-AuNPs would reduce the stability of AuNPs due to the formation of Ag(+) mediated C-Ag(+)-C base pairs accompanied with the AuNPs color change from red to purple or even to dark blue. Moreover, the presence of mercury(II) ions (Hg(2+)) would also reduce the stability of AgNPs due to the formation of Hg(2+) mediated T-Hg(2+)-T base pairs accompanied with the AgNPs color change from yellow to brown, then to dark purple. The presence of both Ag(+) and Hg(2+) will reduce the stability of both AuNPs and AgNPs and cause the visible color change. As a result, Ag(+) and Hg(2+) could be detected qualitatively and quantitatively by the naked eye or by UV-vis spectral measurement. The lowest detectable concentration of a 5 nM mixture of Ag(+) and Hg(2+) in the river water was gotten by the UV-vis spectral measurement.     

Colorimetric detection of Cd2+ using gold nanoparticles cofunctionalized with 6-mercaptonicotinic acid and L-cysteine

We have developed a colorimetric assay for the highly sensitive and selective detection of Cd(2+) using gold nanoparticles (AuNPs) cofunctionalized with 6-mercaptonicotinic acid (MNA) and L-Cysteine (L-Cys) through the formation of an Au-S bond. In the presence of Cd(2+), the aggregation of functionalized AuNPs occurred by means of a metal-ligand interaction that led to visible color changes. Most importantly, cofunctionalized AuNPs had better responses for Cd(2+) than that functionalized by either MNA or L-Cys. Cd(2+) could be detected by the colorimetric response of AuNPs that could be detected by the naked eye or a UV-vis spectrophotometer. The absorbance ratio (A(620)/A(523)) was linear with the Cd(2+) concentration in the range of 2.0 × 10(-7) to 1.7 × 10(-6) M. Under optimum conditions (2.0 × 10(-5) M MNA, 2.0 × 10(-6) M L-Cys and 0.020 M NaCl at pH 10.0), the detection limit (3σ) of Cd(2+) could be as low as 1.0 × 10(-7) M. Interference experiments showed that Pb(2+) and Cu(2+) caused a slight interference for Cd(2+) determination while other metal ions caused no interference. The proposed method was successfully applied to determine the concentration of Cd(2+) in environmental samples (lake water).     

Label-free colorimetric sensing of cobalt(II) based on inducing aggregation of thiosulfate stabilized gold nanoparticles in the presence of ethylenediamine

As a sensitive and selective analytical technique, gold nanoparticles-based colorimetric sensing was characterized by its simplicity and cost-effectiveness. Specific methods have been extensively developed for different targets in diverse samples. In this study, a label-free method for sensing Co(2+) in aqueous solutions was described. The target was achieved by the induced aggregation of thiosulfate (S(2)O(3)(2-)) stabilized gold nanoparticles (AuNPs) in the presence of ethylenediamine (en). Co(2+) first reacted with en and formed complexes of Co(en)(3)(2+) in aqueous solutions, which was followed by the oxidation of Co(en)(3)(2+) to Co(en)(3)(3+) by dissolved oxygen. Co(en)(3)(3+) then attacked S(2)O(3)(2-) ligands adsorbed on the AuNPs' surfaces, forming positively charged (en)(2)CoS(2)O(3)(+) on the AuNPs' surfaces, which reduced the surface charges of AuNPs and induced the aggregation of AuNPs. The process was accompanied by a red-shift in the adsorption spectrum and a visible colour change from wine red to blue. Potential effects of relevant experimental conditions, including pH, concentrations of S(2)O(3)(2-) and en, and incubation time were evaluated for optimization of the method. The proposed method is sensitive (LOD = 0.0 4 μM or 2.36 ppb) and selective (by at least 100-fold over other metal ions except for Cu(2+)) toward Co(2+) with a linear range from 0.1 to 0.7 μM. The cost-effective method allows rapid and simple determination of the concentrations of Co(2+) ions in drinking water.     

Hg2+-mediated aggregation of gold nanoparticles for colorimetric screening of biothiols

In this work, we report a colorimetric assay for the screening of biothiols including glutathione (GSH), cysteine (Cys), and homocysteine (Hcys) based on Hg(2+)-mediated aggregation of gold nanoparticles (AuNPs). Hg(2+) can induce aggregation of thiol-containing naphthalimide (1) capped AuNPs due to the cross-linking interactions from the resulting "thymine-Hg(2+)-thymine" (T-Hg(2+)-T) analogous structure. When Hg(2+) is firstly treated with biothiols, followed by mixing with 1-capped AuNPs suspension, AuNPs undergo a transformation from an aggregation to a dispersion state depending on the concentration of biothiols. This anti-aggregation or re-dispersion of AuNPs is due to the higher affinity of Hg(2+) for biothiols relative to compound 1. The corresponding color variation in the process of anti-aggregation of AuNPs can be used for the quantitative screening of biothiols through UV-vis spectroscopy or by the naked eye. Under optimized conditions, a good linear relationship in the range of 0.025-2.28 μM is obtained for GSH, 0.035-1.53 μM for Cys, and 0.040-2.20 μM for Hcys. The detection limits of this assay for GSH, Cys, and Hcys are 17, 9, and 18 nM, respectively. This colorimetric assay exhibits a high selectivity and sensitivity with tunable dynamic range. The proposed method has been successfully used in the determination of total biothiol content in human urine samples.     

Highly sensitive and selective colorimetric sensors for uranyl (UO2(2+)): development and comparison of labeled and label-free DNAzyme-gold nanoparticle systems

Colorimetric uranium sensors based on uranyl (UO2(2+)) specific DNAzyme and gold nanoparticles (AuNP) have been developed and demonstrated using both labeled and label-free methods. In the labeled method, a uranyl-specific DNAzyme was attached to AuNP, forming purple aggregates. The presence of uranyl induced disassembly of the DNAzyme functionalized AuNP aggregates, resulting in red individual AuNPs. Once assembled, such a "turn-on" sensor is highly stable, works in a single step at room temperature, and has a detection limit of 50 nM after 30 min of reaction time. The label-free method, on the other hand, utilizes the different adsorption properties of single-stranded and double-stranded DNA on AuNPs, which affects the stability of AuNPs in the presence of NaCl. The presence of uranyl resulted in cleavage of substrate by DNAzyme, releasing a single stranded DNA that can be adsorbed on AuNPs and protect them from aggregation. Taking advantage of this phenomenon, a "turn-off" sensor was developed, which is easy to control through reaction quenching and has 1 nM detection limit after 6 min of reaction at room temperature. Both sensors have excellent selectivity over other metal ions and have detection limits below the maximum contamination level of 130 nM for UO2(2+) in drinking water defined by the U.S. Environmental Protection Agency (EPA). This study represents the first direct systematic comparison of these two types of sensor methods using the same DNAzyme and AuNPs, making it possible to reveal advantages, disadvantages, versatility, limitations, and potential applications of each method. The results obtained not only allow practical sensing application for uranyl but also serve as a guide for choosing different methods for designing colorimetric sensors for other targets.     

N-1-(2-mercaptoethyl)thymine modification of gold nanoparticles: a highly selective and sensitive colorimetric chemosensor for Hg2+

An approach for mercury ions (Hg(2+)) sensing based on the Hg(2+)-induced aggregation of thymine (T)-SH-functionalized gold nanoparticles (AuNPs) has been reported. The T-SH ligands that we synthesized can easily be coupled to the surface of AuNPs through the Au-S bond and can recognize Hg(2+) with high selectivity by forming a T-Hg-T complex with strong affinity. For the T-SH-functionalized AuNPs (T-S-AuNPs) sensor, upon addition of Hg(2+), the formation of the T-Hg-T complex induces aggregation of T-S-AuNPs and results in a significant change of color and UV-Vis absorption spectra. Thus, our method can be used for the rapid, easy and reliable screening of Hg(2+) in aqueous solution, with high sensitivity (2.8 nM) and selectivity over competing analytes. The developed method is successfully applied to the sensing of Hg(2+) in real environmental samples.     

A novel colorimetric aptasensor for detection of chloramphenicol based on lanthanum ion–assisted gold nanoparticle aggregation and smartphone imaging

A label-free, rapid response colorimetric aptasensor for sensitive detection of chloramphenicol (CAP) was proposed, which was based on the strategy of ssDNA-modified gold nanoparticle (AuNP) aggregation assisted by lanthanum (La³⁺) ions. The AuNPs generated a color change that could be monitored in the red, green, and blue and analyzed by the smartphone imaging app. La³⁺, as a trigger agent, strongly combined with the phosphate groups of the surface of ssDNA-AuNPs probe, which helps create AuNP aggregation and the color change of AuNPs from red to blue. On the contrary, when mixing with CAP, the aptamer (Apt) bound to CAP to form a rigid structure of the Apt-CAP complex, and La³⁺ attached to the phosphate groups of the complex, which prevented the aptamer from binding to the surface of the AuNPs. As a result, the color of the AuNPs changed to violet-red. Finally, UV-vis absorption spectroscopy and the smartphone imaging app were employed to determine CAP with a lower detection limit of 7.65 nM and 5.88 nM, respectively. The proposed strategy featuring high selectivity and strong anti-interference ability for detection of CAP in practical samples was achieved. It is worth mentioning that the simple and portable colorimetric aptasensor will be used for facilitating on-site detection of food samples.

     

A highly sensitive sensor for Cu2+ with unmodified gold nanoparticles and DNAzyme by using the dynamic light scattering technique

Copper ion (Cu(2+)) plays an important role in many biological reactions, and a suitable level of Cu(2+) is necessary for the regular metabolism of life. Thus developing a sensitive and simple method for determination of Cu(2+) is essential. Here, a novel and sensitive Cu(2+) sensor was developed based on detecting the average hydrodynamic diameter of AuNPs by using dynamic light scattering (DLS). Cu(2+)-specific DNAzyme was double-strand and could not adsorb on the surface of AuNPs, accordingly AuNPs aggregation would occur with the addition of NaCl. However, Cu(2+) could cleave DNAzyme and release single-stranded DNA (ssDNA) fragments, which could adsorb on the surface of AuNPs and prevent them from aggregation. Such differences in DNA adsorption ability on AuNPs before and after the addition of Cu(2+) affected the disperse state of AuNPs directly, and then affected their average hydrodynamic diameter, which could be detected with the DLS technique. Based upon the above mentioned principle, detection of Cu(2+) could be realized over the range from 100 pM to 2.0 nM, with a linear regression equation of D = 306.73 - 89.66C (C: nM, R = 0.9953) and a detection limit of 60 pM (3δ/slope). Moreover, satisfactory results were obtained when the assay was applied in the detection of Cu(2+) in water samples.     

Simple and sensitive detection of dopamine in the presence of high concentration of ascorbic acid using gold nanoparticles as colorimetric probes

A highly sensitive and selective method is presented for colorimetric determination of dopamine using gold nanoparticles (AuNPs). Dopamine induces the aggregation of AuNPs, this resulting in a color change from red to blue or purple. Aggregation is accelerated by the presence of Cu(II), especially at low concentrations of dopamine. The concentration of dopamine can be quantified visually or using a UV-vis spectrometer. The detection limit is as low as 30 nM. The assay is simple, inexpensive, and highly sensitive. Ascorbic acid in even 100-fold molar excess does not interfere. The mechanism of the aggregation of the AuNPs is discussed.     

Gold nanoparticle probes for the detection of mercury, lead and copper ions

Monitoring the levels of potentially toxic metal (PTM) ions (e.g., Hg(2+), Pb(2+), Cu(2+)) in aquatic ecosystems is important because these ions can have severe effects on human health and the environment. Gold (Au) nanomaterials are attractive sensing materials because of their unique size- and shape-dependent optical properties. This review focuses on optical assays for Hg(2+), Pb(2+), and Cu(2+) ions using functionalized Au nanomaterials. The syntheses of functionalized Au nanomaterials are discussed. We briefly review sensing approaches based on changes in absorbance resulting from metal ion-induced aggregation of Au nanoparticles (NPs) or direct deposition of metal ions onto Au NPs. The super-quenching properties of Au NPs allow them to be employed in 'turn on' and 'turn off' fluorescence approaches for the sensitive and selective detection of Hg(2+), Pb(2+), and Cu(2+) ions. We highlight approaches based on fluorescence quenching through analyte-induced aggregation or the formation of metallophilic complexes of Au nanodots (NDs). We discuss the roles of several factors affecting the selectivity and sensitivity of the nanosensors toward the analytes: the size of the Au nanomaterial, the length and sequence of the DNA or the nature of the thiol, the surface density of the recognition ligand, and the ionic strength and pH of the buffer solution. In addition, we emphasize the potential of using new nanomaterials (e.g., fluorescent silver nanoclusters) for the detection of PTM ions.     

Visual and on-site detection of mercury(II) ions on lateral flow strips using DNA-functionalized gold nanoparticles

A test strip for detection of Hg(2+) in aqueous solution based on the DNA-functionalized gold nanoparticles (DNA-AuNPs) was developed and evaluated. When Hg(2+) ions were introduced, the biotinylated DNA(2) hybridized with thiolated DNA(1) functionalized on the AuNPs (DNA(1)-AuNPs) to form mismatch complexes through thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination. The formed mismatch complexes and excess DNA(1)-AuNPs could be captured on the test line formed by streptavidin and the control line formed by DNA(3)-BSA, respectively. Two red lines appeared due to the accumulation of AuNPs, enabling visual detection of Hg(2+) with a detection limit of about 6 nM. The assay results can be obtained within 5 min. The results show that the test strip has excellent sensitivity and selectivity for detection of Hg(2+); thus it holds a great potential for rapid, on-site and real time detection of Hg(2+).     

Colorimetric assay for mercury (II) based on mercury-specific deoxyribonucleic acid-functionalized gold nanoparticles

A colorimetric nanoprobe-mercury-specific DNA-functionalized gold nanoparticles (Au-MSD) was developed for sensing Hg(2+). The new mercury-sensing concept relies on measuring changes in the inhibition of "non-crosslinking" aggregation of Au-MSD-induced by the folding of mercury-specific DNA strand through the thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination. In the absence of Hg(2+), a high concentration of MgCl(2) (50 mM) results in a rapid aggregation of Au-MSD because of the removal of charge repulsion. When Hg(2+) is present, the particles remain stable due to the folding of MSD functionalized on the particle surface. The assay enables the colorimetric detection of Hg(2+) in the concentration range of 0.1-10 μM Hg(2+) ions with a detection limit of 60 nM, and allows for the selective discrimination of Hg(2+) ions from the other competitive metal ions. Toward the goal for practical applications, the sensor was further evaluated by monitoring Hg(2+) in fish tissue samples.     

甲烷氧化菌素功能化金纳米层层自组装修饰电极上过氧化氢的催化还原

借助巯基试剂,在纳米金颗粒表面修饰生物活性物质Mb,制备保持有Mb生物活性的功能化金纳米巯基乙胺-Au NPs-Mb.采用UV-Vis、FTIR光谱和投射电镜表征其结构,该纳米颗粒分布均匀且粒径均一,并显著改善了金纳米颗粒团聚现象.以Mb功能化金纳米为基元,采用单层自组装及层层自组装方式将其修饰到裸金电极表面.各Mb或Mb-Cu电极的电化学测试并未借助电子传递媒介.配位Cu~(2+)后,修饰有Mb的单层及层层自组装修饰的催化还原能力均显著提升.其中Cu~(2+)配位的{巯基乙胺-Au NPs-Mb}3/Au修饰电极作为一种新型H2O2生物传感器,响应时间大约为2 s,米氏常数KappM为0.787 mmol/L,表现出了较强的还原H2O2的催化活性,且稳定性较好.     

A sensitive and rapid UV–vis spectrophotometry for organophosphorus pesticides detection based on Ytterbium (Yb3+) functionalized gold nanoparticle

AuNPs possess oxygen-containing functional groups and strong complexation reaction with Yb3+. While oxygen-containing thiophosphate in the OPs molecule can combine with Yb³⁺ as a cross-linking molecule to produce insoluble yetterbium phosphate, resulting in the aggregation of AuNPs and great decrease in ultraviolet absorbance strength at 520 nm by ultraviolet visible (UV-vis) spectrophotometer.     

Amperometric hydrogen peroxide biosensor based on the immobilization of heme proteins on gold nanoparticles-bacteria cellulose nanofibers nanocomposite

A novel matrix, gold nanoparticles-bacterial cellulose nanofibers (Au-BC) nanocomposite was developed for enzyme immobilization and biosensor fabrication due to its unique properties such as satisfying biocompatibility, good conductivity and extensive surface area, which were inherited from both gold nanoparticles (AuNPs) and bacterial cellulose nanofibers (BC). Heme proteins such as horseradish peroxidase (HRP), hemoglobin (Hb) and myoglobin (Mb) were successfully immobilized on the surface of Au-BC nanocomposite modified glassy carbon electrode (GCE). The immobilized heme proteins showed electrocatalytic activities to the reduction of H₂O₂ in the presence of the mediator hydroquinone (HQ), which might be due to the fact that heme proteins retained the near-native secondary structures in the Au-BC nanocomposite which was proved by UV-vis and IR spectra. The response of the developed biosensor to H₂O₂ was related to the amount of AuNPs in Au-BC nanocomposite, indicating that the AuNPs in BC network played an important role in the biosensor performance. Under the optimum conditions, the biosensor based on HRP exhibited a fast amperometric response (within 1 s) to H₂O₂, a good linear response over a wide range of concentration from 0.3 μM to 1.00 mM, and a low detection limit of 0.1 μM based on S/N = 3. The high performance of the biosensor made Au-BC nanocomposite superior to other materials as immobilization matrix.     

Label free colorimetric sensing of thiocyanate based on inducing aggregation of Tween 20-stabilized gold nanoparticles

Based on inducing the aggregation of gold nanoparticles (AuNPs), a simple colorimetric method with high sensitivity and selectivity was developed for the sensing of thiocyanate (SCN(-)) in aqueous solutions. Citrate-capped AuNPs were prepared following a classic method and Tween 20 was subsequently added as a stabilizer. With the addition of SCN(-), citrate ions on AuNPs surfaces were replaced due to the high affinity between SCN(-) and Au. As a result, Tween 20 molecules adsorbed on the AuNPs surfaces were separated and the AuNPs aggregated. The process was accompanied by a visible color change from red to blue within 5 min. The sensing of SCN(-) can therefore be easily achieved by a UV-vis spectrophotometer or even by the naked eye. The potential effects of relevant experimental conditions, including concentration of Tween 20, pH, incubation temperature and time, were evaluated to optimize the method. Under optimized conditions, this method yields excellent sensitivity (LOD = 0.2 μM or 11.6 ppb) and selectivity toward SCN(-). Our attempt may provide a cost-effective, rapid and simple solution to the inspection of SCN(-) ions in saliva and environmental aqueous samples.     

Aggregation and interaction of cationic nanoparticles on bacterial surfaces

Cationic monolayer-protected gold nanoparticles (AuNPs) with sizes of 6 or 2 nm interact with the cell membranes of Escherichia coli (Gram-) and Bacillus subtilis (Gram+), resulting in the formation of strikingly distinct AuNP surface aggregation patterns or lysis depending upon the size of the AuNPs. The aggregation phenomena were investigated by transmission electron microscopy and UV-vis spectroscopy. Upon proteolytic treatment of the bacteria, the distinct aggregation patterns disappeared.     

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

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

Gold nanoparticles-based colorimetric investigation of triplex formation under weak alkalic pH environment with the aid of Ag+

A novel colorimetric method for investigating triplex formation between oligonucleotide modified Au nanoparticles (AuNPs) under weak alkalic pH environment is developed based upon the specific recognition property of Ag+ with CGC triads. Oligonucleotide 5'-SH-T12-CTTCTTTCCTTTCTTC-3' (oligo-1) is modified on the surface of AuNPs. Upon addition of oligonucleotide 5'-GAAGAAAGGAAAGAAG-3' (oligo-2), triplex formation between oligo-1 modified AuNPs occurred at pH 8.0 with the aid of Ag+, triggers the aggregation of AuNPs, accompany with the solution color change from red to purple. The melting temperature demonstrates a 31 °C increase for the triplex DNA compose of 10 T?A°T triads and 6 C?G°C triads upon addition of Ag+, the disassociation constant (Kd) between Ag+ and C?G°C triads is 3.6 μM. Moreover, triplex formation between AuNPs depending on Ag+ can be used to recognize Ag+ ion with the naked eye, as well as UV-vis absorption spectroscopy.     

Coordination of mercury(II) to gold nanoparticle associated nitrotriazole towards sensitive colorimetric detection of mercuric ion with a tunable dynamic range

A simple colorimetric assay with high sensitivity, excellent selectivity and a tunable dynamic range is reported for detecting trace amounts of mercuric ion in aqueous solution based on the coordination of Hg(2+) to the gold nanoparticle (AuNP)-associated 3-nitro-1H-1,2,4-triazole (NTA). The NTA can stabilize the AuNPs against tris-induced aggregation through capping the AuNPs. In the presence of Hg(2+), the NTA is released from the AuNP surface via the formation of a NTA-Hg(2+) coordination complex, leading to the aggregation of AuNPs in tris. This detection strategy is unique in terms of high sensitivity and excellent selectivity, a tunable dynamic range, and simplicity of probe preparation. Low detection limits of 7 nM (1.4 ppb) and 50 nM (10 ppb) can be achieved by spectrophotometer and by direct visualization, respectively, under the optimized conditions. No noticeable colour changes are observed towards other metal ions (Ag(+), Zn(2+), Ni(2+), Cr(3+), Mg(2+), Cu(2+), Co(2+), Cd(2+), Pb(2+), Fe(2+)) at concentrations up to 100 μM without the need of any other masking agents. In addition, the dynamic range of the assay can be easily tuned by adjusting the amount of NTA in the NTA-AuNP probes. More importantly, the NTA-AuNP probes can be simply prepared by mixing NTA with as-synthesized citrate-capped AuNPs. This not only avoids complicated surface modifications and tedious separation processes, but also is cost-effective.