Haijuan Li

Selective Synthesis of Single-Crystalline Rhombic Dodecahedral, Octahedral, and Cubic Gold Nanocrystals

This paper reports a versatile seed-mediated growth method for selectively synthesizing single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals. In the seed-mediated growth method, cetylpyridinium chloride (CPC) and CPC-capped single-crystalline gold nanocrystals 41.3 nm in size are used as the surfactant and seeds, respectively. The CPC-capped gold seeds can avoid twinning during the growth process, which enables us to study the correlations between the growth conditions and the shapes of the gold nanocrystals. Surface-energy and kinetic considerations are taken into account to understand the formation mechanisms of the single-crystalline gold nanocrystals with varying shapes. CPC surfactants are found to alter the surface energies of gold facets in the order {100} > {110} > {111} under the growth conditions in this study, whereas the growth kinetics leads to the formation of thermodynamically less favored shapes that are not bounded by the most stable facets. The competition between AuCl(4)(-) reduction and the CPC capping process on the {111} and {110} facets of gold nanocrystals plays an important role in the formation of the rhombic dodecahedral (RD) and octahedral gold nanocrystals. Octahedral nanocrystals are formed when the capping of CPC on {111} facets dominates, while RD nanocrystals are formed when the reduction of AuCl(4)(-) on {111} facets dominates. Cubic gold nanocrystals are formed by the introduction of bromide ions in the presence of CPC. The cooperative work of cetylpyridinium and bromide ions can stabilize the gold {100} facet under the growth condition in this study, thereby leading to the formation of cubic gold nanocrystals.

Simultaneous electrochemical determination of uric acid, dopamine, and ascorbic acid at single-walled carbon nanohorn modified glassy carbon electrode

Single-walled carbon nanohorn modified glassy carbon electrode (SWCNH-modified GCE) was first employed for the simultaneous determination of uric acid (UA), dopamine (DA), and ascorbic acid (AA). The SWCNH-modified GCE displayed excellent electrochemical catalytic activities. The oxidation overpotentials of UA, DA, and AA decrease significantly and their oxidation peak currents increase dramatically at SWCNH-modified GCE. Linear sweep voltammetry (LSV) was used for the simultaneous determination of UA, DA, and AA in their ternary mixture. The peak separations between UA and DA, and DA and AA are large up to 152mV and 221mV, respectively. The calibration curves for UA, DA, and AA were obtained in the range of 0.06-10 microM, 0.2-3.8microM, and 30-400microM, respectively. The detection limits (S/N=3) were 20nM, 60nM, and 5microM for UA, DA, and AA, respectively. The proposed method improved sensitivity for the determination of UA by more than one order of magnitude. The present method was applied to the determination of UA, DA, and AA in urine sample by using standard adding method and the results were satisfactory.

Amperometric glucose biosensor based on single-walled carbon nanohorns.

The biosensing application of single-walled carbon nanohorns (SWCNHs) was demonstrated through fabrication of an amperometric glucose biosensor. The biosensor was constructed by encapsulating glucose oxidase in the Nafion-SWCNHs composite film. The cyclic voltammograms for glucose oxidase immobilized on the composite film displayed a pair of well-defined and nearly symmetric redox peaks with a formal potential of -0.453 V. The biosensor had good electrocatalytic activity toward oxidation of glucose. To decrease detection potential, ferrocene monocarboxylic acid was used as a redox mediator. The mediated glucose biosensor shows a linear range from 0 to 6.0 mM. The biosensor shows high sensitivity (1.06 microA/mM) and stability, and can avoid the commonly coexisted interference. Because of impressive properties of SWCNHs, such as high purity and high surface area, SWCNHs and their composites are expected to be promising material for biomolecular immobilization and biosensing applications.

[Ru(bpy)2dppz]2+ Electrochemiluminescence Switch and Its Applications for DNA Interaction Study and Label-free ATP Aptasensor

[Ru(bpy)2dppz]2+ electrochemiluminescence (ECL) was studied, and it was used to investigate DNA interaction and develop a label-free ATP aptasensor for the first time. ECL of [Ru(bpy)2dppz]2+ is negligible in aqueous solution, and increases approximately 1000 times when [Ru(bpy)2dppz]2+ intercalates into the nucleic acid structure. The ECL switch behavior of [Ru(bpy)2dppz]2+ is ascribed to the intercalation that shields the phenazine nitrogens from the solvent and results in a luminescent excited state. The ECL switch by DNA was applied to investigate the interaction of [Ru(bpy)2dppz]2+ with herring sperm DNA. The calculated equilibrium constant (K) is 1.35 x 10(6) M(-1), and the calculated binding-site size (s) is 0.88 base pair, which is consistent with the reported values. Moreover, ATP can dramatically affect ECL of the [Ru(bpy)2dppz]2+/ATP aptamer complex. As a result, a label-free, sensitive, and selective [Ru(bpy)2dppz]2+ ECL method for ATP detection was developed. The detection limit is 100 nM for ATP (with a signal-to-noise ratio, S/N, of 3) with a linear range of 0-1 microM. The result demonstrates that [Ru(bpy)2dppz]2+ ECL holds great promise in aptasensors.

Electrochemiluminescence from tris(2,2′-bipyridyl)ruthenium(II)–graphene–Nafion modified electrode

An electrochemiluminescence (ECL) sensor based on Ru(bpy)(3)(2+)-graphene-Nafion composite film was developed. The graphene sheet was produced by chemical conversion of graphite, and was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and Raman spectroscopy. The introduction of conductive graphene into Nafion not only greatly facilitates the electron transfer of Ru(bpy)(3)(2+), but also dramatically improves the long-term stability of the sensor by inhibiting the migration of Ru(bpy)(3)(2+) into the electrochemically inactive hydrophobic region of Nafion. The ECL sensor gives a good linear range over 1x10(-7) to 1x10(-4)M with a detection limit of 50 nM towards the determination of tripropylamine (TPA), comparable to that obtained by Nafion-CNT. The ECL sensor keeps over 80% and 85% activity towards 0.1 mM TPA after being stored in air and in 0.1M pH 7.5 phosphate buffer solution (PBS) for a month, respectively. The long-term stability of the modified electrode is better than electrodes modified with Nafion, Nafion-silica, Nafion-titania, or sol-gel films containing Ru(bpy)(3)(2+). Furthermore, the ECL sensor was successfully applied to the selective and sensitive determination of oxalate in urine samples.

Single-walled carbon nanohorn as new solid-phase extraction adsorbent for determination of 4-nitrophenol in water sample.

Single-walled carbon nanohorn (SWCNH) was developed as new adsorbent for solid-phase extraction using 4-nitrophenol as representative. The unique exoteric structures and high surface area of SWCNH allow extracting a large amount of 4-nitrophenol over a short time. Highly sensitive determination of 4-nitrophenol was achieved by linear sweep voltammetry after only 120s extraction. The calibration plot for 4-nitrophenol determination is linear in the range of 5.0x10(-8) M-1.0x10(-5) M under optimum conditions. The detection limit is 1.1x10(-8) M. The proposed method was successfully employed to determine 4-nitrophenol in lake water samples, and the recoveries of the spiked 4-nitrophenol were excellent (92-106%).

Hydrogen peroxide biosensor based on direct electrochemistry of soybean peroxidase immobilized on single-walled carbon nanohorn modified electrode.

Single-walled carbon nanohorns (SWCNHs) were used as a novel and biocompatible matrix for fabricating biosensing devices. The direct immobilization of acid-stable and thermostable soybean peroxidase (SBP) on SWCNH modified electrode surface can realize the direct electrochemistry of enzyme. Cyclic voltammogram of the adsorbed SBP displays a pair of redox peaks with a formal potential of -0.24 V in pH 5 phosphate buffer solution. The formal potential has a linear relationship with pH from 3 to 9 with a slope of -48.7 mV/pH, close to the value of -55.7 mV/pH expected at 18 degrees C for the reversible transfer of one proton and one electron. Bioactivity of SBP remains good in SWCNH microenvironment, along with effective catalysis of the reduction of H(2)O(2). In the absence of a mediator, this H(2)O(2) biosensor exhibited a high sensitivity (16.625 microAL/mmol), a linear range from 0.02 to 1.2 mmolL(-1), and a detection limit of 5.0 x 10(-7) mmolL(-1), as well as acceptable preparation reproducibility and excellent stability.

Label-free and signal-on electrochemiluminescence aptasensor for ATP based on target-induced linkage of split aptamer fragments by using [Ru(phen)3]2+ intercalated into double-strand DNA as a probe.

Aptamers are specific nucleic acids (DNA or RNA) selected from random sequence libraries using SELEX (systemic evolution of ligands by exponential enrichment) with high affinity and specificity in the binding to small molecules, proteins and other macromolecules. Owing to their simple synthesis, good stability, easy storage, and simple modification for further immobilization procedure, aptamers have attracted increasing interest as the ideal recognition elements for biosensor applications. Sandwich-type assays have been widely used in biosensors for their specificity and low detection limits. However, the approach largely excluded aptamer-based sensors due to the requirement that the target exposes two distinct epitopes. Hence, rather few aptamer-based sandwich-type assays have been reported for proteins, much less small molecules. Recently, the construction of aptasensors for small molecules based on linkage of split-aptamer fragments, in the presence of the analyte-substrate, creating a “sandwich assay”, was introduced as a general platform for aptasensors. However, most aptasensors need chemical labeling procedures, which are usually complex, timeconsuming, and labor-intense. Therefore it is desirable to establish a label-free aptamer-based sandwich-type assay with high specificity and low detection limit for small molecules. It has been reported that the double-strand DNA (dsDNA) has the capacity to be intercalated with some small molecules into its grooves with high affinity; some aptamer-based sensors have been developed based on the intercalation of small molecule probes into the DNA structures. Nevertheless, most of these sensors are based on the competing reaction between the analytes and complementary strands, which might be more difficult than only target–aptamer interaction. The factor could lead to relatively slow response to the target compared with some noncompetition assays. Recently, electrochemiluminescence (ECL) aptasensors, which integrate the advantages of electrochemical detection and chemiluminescent techniques, have received particular attention due to their high sensitivity and selectivity, wide linear ranges, as well as low production cost. As a popular ECL reagent with high ECL emission efficiency for bioassays, RuACHTUNGTRENNUNG(phen)32+ (phen =1,10-phenanthroline) can intercalate into the grooves of dsDNA. In comparison to the commonly used DNA-binding fluorescing intercalator ethidium bromide, Ru ACHTUNGTRENNUNG(phen)32+ is more expensive, but has the advantages of lower toxicity, better stability, and easy use. Moreover, RuACHTUNGTRENNUNG(phen)32+ can be used not only in fluorescence study but also ECL studies, which do not require expensive light source as in fluorescence study. Herein, a label-free sandwich-type ECL sensing system based on target-induced conjunction of split aptamer fragments has been developed by the use of Ru ACHTUNGTRENNUNG(phen)32+ intercalated into ds-DNA as the ECL probe. ATP was selected as the model target to demonstrate the principle of the present ECL aptamer-based assay. The design concept of the sensing system and the ATP detection are displayed in Scheme 1. To assemble the aptasensor, the 27-mer anti-ATP DNA aptamer was divided into two different fragments which do not interact with each other in the absence of ATP. One of them, modified with thiol at 5’ terminus (1), was immobi[a] M. Sc. Z. Liu, M. Sc. W. Zhang, B. Sc. L. Hu, B. Sc. H. Li, M. Sc. S. Zhu, Prof. Dr. G. Xu State Key Laboratory of Electroanalytical Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences, Changchun, Jilin 130022 (China) Fax: (+86) 431-85262747 E-mail : guobaoxu@ciac.jl.cn [b] B. Sc. L. Hu, B. Sc. H. Li, M. Sc. S. Zhu Graduate University of the Chinese Academy of Sciences Chinese Academy of Sciences, Beijing 100864 (China)

Immobilization of tris(1,10-phenanthroline)ruthenium with graphene oxide for electrochemiluminescent analysis

Electrochemiluminescence (ECL) of ruthenium complexes has broad applications and the immobilization of Ru(bpy)(3)(2+) has received extensive attention. In comparison with Ru(bpy)(3)(2+), Ru(phen)(3)(2+) can be immobilized more easily because of its better adsorbability. In this study, immobilization of Ru(phen)(3)(2+) for ECL analysis has been demonstrated for the first time by using graphene oxide (GO) as an immobilization matrix. The immobilization of Ru(phen)(3)(2+) is achieved easily by mixing Ru(phen)(3)(2+) with GO without using any ion exchange polymer or covalent method. The strong binding of Ru(phen)(3)(2+) with GO is attributed to both the π-π stacking interaction and the electrostatic interaction. The Ru(phen)(3)(2+)/GO modified electrode was characterized by using tripropylamine (TPA) as the coreactant. The linear range of TPA is from 3×10(-7) to 3×10(-2) mol L(-1) with the detection limit of 3×10(-7) mol L(-1). The ECL sensor demonstrates outstanding long-term stability. After the storage in the ambient environment for 90 days, the ECL response remains comparable with its original signal.

Functionalized single-walled carbon nanohorns for electrochemical biosensing

Single-walled carbon nanohorns (SWNHs), distinguished by their high purity and distinct structure, were noncovalently functionalized with poly(sodium 4-styrenesulfonate). The functionalized SWNHs were characterized by scanning electron microscopy, atomic force microscopy, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetry. Heme protein myoglobin was adsorbed onto surface of functionalized SWNHs to prepare electrochemical biosensor. Surface assembly process and direct electrochemistry of immobilized myoglobin were investigated by electrochemical impedance spectroscopy and cyclic voltammetry, respectively. The proposed biosensor exhibited good electrocatalysis to the reduction of hydrogen peroxide. The response was linear over the range 3-350 microM with a detection limit of 0.5 microM. Good reproducibility and stability of the biosensor were obtained toward hydrogen peroxide detection.