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.     

Mixed charged zwitterionic self-assembled monolayers as a facile way to stabilize large gold nanoparticles

Here we report a facile way of stabilizing large gold nanoparticles (AuNPs) by mixed charged zwitterionic self-assembled monolayers (SAMs). The citrate-capped AuNPs with diameters ranging from 16 nm to even ～100 nm are well stabilized via a simple place exchange reaction with a 1:1 molar ratio mixture of negatively charged sodium 10-mercaptodecanesulfonic acid (HS-C10-S) and positively charged (10-mercaptodecyl)-trimethyl-ammonium bromide (HS-C10-N4). The 16 nm AuNPs protected by mixed charged zwitterionic SAMs not only show much better stability than the single negatively or positively charged AuNPs, but also exhibit exciting stability as well as those modified by monohydroxy (1-mercaptoundec-11-yl) tetraethylene glycol (HS-C11-EG4). Importantly, 16 nm AuNPs protected by mixed SAMs exhibit good stability in cell culture medium with 10% FBS and strong protein resistance, especially with excellent resistance against plasma adsorption. Moreover, the mixed charged zwitterionic SAMs are also able to well-stabilize larger AuNPs with a diameter of 50 nm, and to help remarkably improve their stability in saline solution compared with HS-C11-EG4 protected ones. When it comes to AuNPs with a diameter of 100 nm, the mixed charged zwitterionic SAM protected nanoparticles retain a smaller hydrodynamic diameter and even better long-term stability than those modified by mercaptopolyethylene glycol (M(w) = 2000, HS-PEG2000). The above results demonstrated that the mixed charged zwitterionic SAMs are able to have a similar effect on stabilizing the large gold nanoparticles just like the single-component zwitterionic SAMs. Concerning its ease of preparation, versatility, and excellent properties, the strategy based on the mixed charged zwitterionic SAM protection might provide a promising method to surface tailoring of nanoparticles for biomedical application.     

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.     

A novel Surface Plasmon Resonance enhanced Total Internal Reflection Ellipsometric application: electrochemically grafted isophthalic acid nanofilm on gold surface

The scope of this study is to modify a Surface Plasmon Resonance (SPR) sensor slide with isophthalic acid to evaluate the possible application on the detection of copper(II) ions in aqueous media by total internal reflection ellipsometry. A gold sensor surface was modified by an electrochemical diazonium reduction modification method. The modified surfaces are characterized with cyclic voltammetry (CV) and ellipsometry. Isophthalic acid monolayer modified gold slides were used for in situ detection of aqueous Cu(2+) solution with the SPR enhanced total internal reflection ellipsometry (SPRe-TIRE) technique. Layer formation, pH dependency of adsorption, sensor response of the SPRe-TIRE and isothermal kinetic parameters were examined. A high dependency on the number of CV cycles in the monolayer-multiple layer transition was observed. The suggested sensor gave a linear response over a wide range of Cu(2+) concentrations. It was also reported that adsorption on the SPRe-TIRE sensor gave Langmuir adsorption model behavior.     

DNAzyme crosslinked hydrogel: a new platform for visual detection of metal ions

We propose the use of DNAzyme as a crosslinker of hydrogel to develop a catalytic platform for the sensing of metal ions. The DNAzyme crosslinked hydrogel can undergo gel-sol transition in response to Cu(2+) ions, which enables sensitive visual detection of Cu(2+) by observing the release of pre-trapped AuNPs.     

Electrochemical aptasensor for detection of copper based on a reagentless signal-on architecture and amplification by gold nanoparticles

A highly sensitive and specific electrochemical aptasensor for Cu(2+) detection based on gold nanoparticles (AuNPs) is presented. In this work, AuNPs offered a big surface area to immobilize a large number of aptamers and excellent electrochemical signal transduction. Its high sensitivity, low detection limit, and wide detection range are the main advantages over our former copper aptasensor. The peak current increased proportionally to the Cu(2+) concentration over the range from 0.1 nM to 10 μM with a detection limit of 0.1 pM. The presence of other divalent metal ions did not affect the detection of Cu(2+), which indicates a high specificity of Cu(2+) detection could be detected. Rapidity, simplicity, and excellent selectivity make it suitable for practical use in determination of Cu(2+) from lake samples.     

In situ synthesis of gold nanoparticles on porous polyacrylonitrile nanofibers for sensing applications

A simple and cost-effective method was reported to synthesize small size (6 nm) gold nanoparticles (AuNPs) on polyacrylonitrile (PAN) electrospun nanofibers (AuNPs/PAN). The formation of AuNPs is attributed to the in situ reduction of Au(III) to Au(0) by 4-(dimethylamino)benzaldehyde doped in the PAN nanofibers. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) confirmed that the AuNPs/PAN nanofibers showed good conductivity. The AuNPs/PAN nanofibers were used to immobilize tris(2,2'-bipyridyl)ruthenium(II) ions (Ru(bpy)(3)(2+)) to form an electrochemiluminescence (ECL) sensor. The AuNPs on the PAN nanofibers exhibited an excellent catalytic effect on the ECL of Ru(bpy)(3)(2+) which could be employed to detect low concentrations of phenolic compounds. The linear response range of the ECL sensor to hydroquinone is 0.55-37 μM with limit of detection of 80 nM (S/N = 3). This sensor has been successfully applied to determine the hydroquinone content in photographic developer samples. Our work provides a very simple and cost-effective method to synthesize AuNPs on polymer nanofibers which shows great potential in the field of electrocatalysis and chemo/biosensors.     

Selective electroless deposition of copper on organic thin films with improved morphology

We have investigated the selective electroless deposition (ELD) of Cu on functionalized self-assembled monolayers (SAMs). Previous studies have demonstrated that Cu deposits on -COOH and -CH(3) terminated SAMs using ELD. However, the deposited films were rough and contained irregular crystallites. Further, the copper penetrated through the film. In this Article, we demonstrate that copper can be selectively deposited on -COOH terminated SAMs with improved morphology and without penetration of copper through the organic layer. The method employs a Cu(II) seed layer and an additive, adenine or guanine. We demonstrate the efficacy of the technique on photopatterned -CH(3)/-COOH SAMs. Copper is observed to deposit only atop the -COOH terminated SAM area and not on the -CH(3) terminated SAM. The use of a Cu(II) seed layer increased the Cu ELD rate on both -COOH and -CH(3) terminated SAMs. The deposited copper layer strongly adheres to the -COOH terminated SAMs because the copper layer nucleates at Cu(2+)-carboxylate complexes. In contrast, the deposited copper layer can easily be removed from the -CH(3) terminated SAM surface because there is no specific copper-surface interaction. The additives adenine and guanine mediate the interaction of Cu(2+) and the deprotonated -COOH terminated SAMs via the formation of additive-carboxylate complexes. These complexes lead to significantly reduced copper penetration through the SAM. In the case of adenine, the diffusion of copper through the organic film was eliminated. This new technique for copper deposition will facilitate the development of inexpensive molecular electronics, sensors, and other nanotechological devices.     

3D Nano‐structure L‐cysteine/AuNPs/Bi2O3 Modified Glass Carbon Electrode as Chemical Sensor for Highly Sensitive and Selective Detection Cu (II)

It was first time using the l‐cysteine self‐assembled on the surface of gold nanoparticles and Bi₂O₃ nano‐structured materials modified GCE composed l‐cysteine/AuNPs/Bi₂O₃/GCE sensor. The sensor possessed three‐dimensional nanostructure and exhibited a higher ratio of activity sites, large active surface, fast electron transfer rate, excellent catalytic, sensing characteristics and larger affinity to Cu (II). The sensor was determined to have an excellent sensitivity and selectivity for the detection of Cu (II). The characterization of sensor as well as the optimization of the analytical procedure was reported. The optimized conditions parameters allowed the detection of Cu (II) concentration following short analysis time, a detection limit of 5×10^?11 M at 80 s of preconcentration time was obtained using the as‐prepared sensor, and also show excellent stability and good repeatability, and, thus, could be used for detection of Cu (II) in environment.     

2,1,3-Benzoxadiazole-based selective chromogenic chemosensor for rapid naked-eye detection of Hg2+ and Cu2+

We report a simple twisted intramolecular charge transfer (TICT) chromogenic chemosensor for rapid and selective detection of Hg(2+) and Cu(2+). The sensor was composed of an electron-acceptor 4-fluoro moiety and an electron-donor 7-mercapto-2,1,3-benzoxadiazole species where the S together with the 1-N provided the soft binding unit. Upon Hg(2+) and Cu(2+) complexation, remarkable but different absorbance spectra shifts were obtained in CH(3)CN-H(2)O mixed buffer solution at pH 7.6, which can be easily used for naked-eye detection. The sensor formed a stable 2:1 complex with Cu(2+), and both 2:1 and 3:1 complexes with Hg(2+). While alkali-, alkaline earth- and other heavy and transition metal ions such as Na(+), Mg(2+), Mn(2+), Co(2+), Ni(2+), Ag(+), Zn(2+), Pb(2+) and Cd(2+) did not cause any significant spectral changes of the sensor. This finding is not only a supplement to the detecting methods for Hg(2+) and Cu(2+), but also adds new merits to the chemistry of 4,7-substituted 2,1,3-benzoxadiazoles.     

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.     

Self-induced "electroclick" immobilization of a copper complex onto self-assembled monolayers on a gold electrode

We report the self-induced "electroclick" immobilization of the [Cu(II)(6-ethynyl-TMPA)(H(2)O)](2+) complex, by its simple electro-reduction, onto a mixed azidoundodecane-/decane-thiol modified gold electrode. The redox response of the grafted [Cu(II/I)(TMPA)] at the modified electrode is fully reversible indicating no Cu coordination change and a fast electron transfer.     

Two-column ion-exchange method for the determination of copper-complexing capacity and conditional stability constants of copper complexes for ligands in natural waters

Sample solutions titrated with Cu(2+) ions are passed sequentially through two ion-exchange columns in an automated flow system. The first column is packed with Chelex-100 resin and retains Cu(2+) ions that are free or derived from copper complexes that dissociate in the column. The second column is packed with AG MP-1 anion-exchange resin and retains negatively charged Cu(II) complexes. The retained copper species are then eluted from the columns and determined on-line with a flame atomic-absorption spectrophotometer. It is necessary to correct for a small fraction of free Cu(2+) ions that pass through the first column and are retained by the second column. The Cu(II)-complexing capacity of sample solutions is determined from plots of the concentration ratio of free Cu(2+) ions to Cu(II) complexes vs. the concentration of free Cu(2+) ions. Conditional stability constants of the copper complexes are also estimated from these plots. The complexing capacity of sample solutions is also determined rapidly by measuring the concentration of complexed Cu(II) after spiking the sample with an excess of Cu(2+) ions. The sample solutions tested were 4.0muM NTA, 4.0-mg/l. humic acid, and a river water.     

Electroactive dipyrromethene-Cu(II) self-assembled monolayers: complexation reaction on the surface of gold electrodes

In the work presented, thiol- and COOH-terminated dipyrromethene derivatives have been applied for gold electrode modification. Dipyrromethene deposited onto a solid support, after binding Cu2+, can act as a redox active monolayer. The complexation of Cu(II) ions has been performed on the surface of gold electrodes modified with dipyrromethene. The characterization of dipyrromethene-Cu(II) self-assembled monolayers (SAMs) has been done by cyclic voltammetry (CV), wettability contact angle measurements, and atomic force microscopy (AFM). The new electroactive monolayer could be applied for the immobilization of proteins and ssDNA or for electrochemical anion sensing without redox markers in the solution.     

A screen-printed carbon electrode modified with a bismuth film and gold nanoparticles for simultaneous stripping voltammetric determination of Zn(II), Pb(II) and Cu(II)

The authors report on a disposable sensor for the differential pulse anodic stripping voltammetric (DPASV) determination of the ions Zn(II), Pb(II) and Cu(II). Simultaneous detection is accomplished by using a screen-printed carbon electrode (SPCE) co-modified with an in-situ plated bismuth (Bi)) film and gold nanoparticles (AuNPs). The synergistic effect of the Bi film, and the large surface and good electrical conductivity of the AuNPs strongly assist in the co-deposition of the three ions. Four well-defined and fully separated anodic stripping peaks, at 540 mV for Zn(II), 50 mV for Pb(II), 140 mV for Bi(III) and 295 mV for Cu(II), all vs. Ag/AgCl, can be seen. The modified SPCE was characterized by scanning electron microscopy, X-ray diffraction, cyclic voltammetry and electrochemical impedance spectroscopy. Under the optimized conditions, the sensor has a good response to these ions. The detection limits (at an S/N ratio of 3) are 50 ng·L^?1 for Zn(II), 20 ng·L^?1 for Pb(II), and 30 ng·L^?1 for Cu(II). The method was applied to the determination of the 3 ions in spiked lake water samples.

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Graphical abstract Schematic of screen-printed carbon electrode (SPCE) co-modified with a bismuth film and gold nanoparticles for electrochemical simultaneous determination of Zn(II), Pb(II) and Cu(II) by differential pulse anodic stripping voltammetric (DPASV).

     

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).     

Electropolymerized surface ion imprinting films on a gold nanoparticles/single-wall carbon nanotube nanohybrids modified glassy carbon electrode for electrochemical detection of trace mercury(II) in water

Electrochemical detection of Hg(II) using a electropolymerized ion imprinting poly(2-mercaptobenzothiazole) films at the surface of gold nanoparticles/single-walled carbon nanotube nanohybrids modified glassy carbon electrode (PMBT/AuNPs/SWCNTs/GCE) is described for the first time. The Hg(II)-imprinted PMBT/AuNPs/SWCNTs/GCE sensor exhibits larger binding to functionalized capacity, larger affinity, faster binding kinetics and higher selectivity to template Hg(II). The differential pulse anodic stripping voltammetry (DPASV) response of the Hg(II)-imprinted PMBT/AuNPs/SWCNTs/GCE sensor to Hg(II) is ca. 3.7- and 10.5-fold higher than that at the non-imprinted PMBT/AuNPs/SWCNTs/GCE and the imprinted PMBT/AuNPs/GCE, respectively, and the detection limit for Hg(II) is 0.08 nM (S/N = 3, which is well below the guideline value given by the World Health Organization) and a sensitivity of 0.749 μA nM⁻¹ was obtained. Excellent wide linear range (0.4–96.0 nM) and good repeatability (relative standard deviation of 2.6%) were obtained for Hg(II). The interference experiments show that Ag(I), Pb(II), Cd(II), Zn(II) and Cu(II) had little or no influence on the Hg(II) signal. These values, particularly the high sensitivity and excellent selectivity in contrast to the values reported previously in the area of electrochemical Hg(II) detection, demonstrate the analytical performance of the Hg(II)-imprinted PMBT/AuNPs/SWCNTs/GCE toward Hg(II) is superior to the existing electrodes and could be used for efficient determination of Hg(II) in natural water samples.     

界面可控硫醇SAMs纳米金修饰金电极的电化学行为研究

在裸金电极上自组装不同比例的4,4’-二甲基联苯硫醇(MTP)和硫辛酸(TA)混合液,形成自组装膜(MTP+TA/Au SAMs),再修饰纳米金,制得纳米金混合巯基修饰金电极(AuNPs/MTP+TA/Au)。研究了纳米金混合巯基修饰金电极的电化学行为和阻抗行为,结果表明电极表面pH值的改变对电极表面的电子转移有重要影响。对葡萄糖传感器的制备条件、测定条件、抗干扰能力等进行了讨论,结果表明修饰电极的微结构和微环境有必要进一步研究。     

Simultaneous determination of Cu(II), Ni(II) and Zn(II) by peroxyoxalate chemiluminescence using Partial Least Squares calibration

It was found that the ions Cu(II), Ni(II) and Zn(II) can attenuate the peroxyoxalate chemiluminescence emission, which was used to develop an analytical procedure for the simultaneous determination of these ions in a stopped-flow system using Partial Least Square (PLS) calibration. Acetonitrile was used to dissolve TCPO and to prepare a mixture of fluorescein, H(2)O(2) and imidazole. These solutions were carried using two peristaltic pumps, while a third pump was employed to propel the aqueous solutions of the metallic ions. All solutions were mixed in the quartz cell of a Campsec CL detector connected to a personal computer to register the CL development using the Clarity software. Under the optimum operative conditions each ion produced a specific CL development with maximum intensities at 0.280 min for Zn(II), 0.307 min for Ni(II) and 0.327 min for Cu(II). The latter exhibited the highest inhibition effect. The experimental calibration set was composed of 16 sample solutions using a central design for three component mixtures with scaled values. The proposed method offers the advantages of simplicity, good precision and accuracy for the simultaneous determination of Ni(2+), Cu(2+) and Zn(2+) in water samples.     

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.