Effect of monovalent cations (Li+, Na+, K+, Cs+) on self-assembly of thiol-modified double-stranded and single-stranded DNA on gold electrode.

In this article we investigate the effect of monovalent cations (Li(+), Na(+), K(+), Cs(+)) on self-assembly of thiol-modified double-stranded DNA (ds-DNA) and single-stranded DNA (ss-DNA) on gold electrodes. Electrochemical characteristics (surface coverage, ion penetration and charge transfer) of ds-DNA and ss-DNA self-assembled monolayers (SAMs) formed with different monovalent cations are inspected based on six important interfacial parameters including surface coverage (Γ(m)), interfacial capacitance (C), phase angle (Φ(1 Hz)), ion transfer resistance (R(it)*), current density difference (Δj) and charge transfer resistance (R(ct)) from chronocoulometry (CC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Three sections are included: (1) Investigation of the relationships of parameters (Γ(m), C, Φ(1 Hz), R(it)*, Δj and R(ct)) for ds-DNA-SAMs and ss-DNA-SAMs with cation types and concentrations; (2) confirmation and explanation of our experimental results combined with our recently proposed simple DNA model and literature reports; (3) exploration of the mechanism for the orders of monovalent cations (Li(+), Na(+), K(+), Cs(+)) on availing the adsorption of ds-DNA and ss-DNA molecules on gold based on their physicochemical parameters (ion size, solvation free energy and enthalpy, ion-water bond length and water exchange rate) and possible binding modes with DNA molecules. This work might provide a useful reference for understanding interactional mechanism of cations with DNA molecules.

Studies on the effect of electrode pretreatment on the coverage of self-assembled monolayers of dodecanethiol on gold by electrochemical reductive desorption determination.

This paper examines the effect of five major pretreatments on the surface coverage Γ(m) of dodecanethiol self-assembled monolayer on polycrystalline gold electrode (C(12)SH-SAMs-Au). It is based on the electrochemical reductive desorption in the alkaline solution by cyclic voltammetry (CV). The five different pretreatment methods include: aqua regia pretreatment, reductive annealed pretreatment, UV/O(3) pretreatment, piranha reagents pretreatment and simple polishing pretreatment, and then all above pretreatments following the same procedure of electrochemistry cleaning. The experimental results show that the surface coverage Γ(m) for C(12)SH-SAMs-Au by the five pretreatment methods are: aqua regia pretreatment (8.0 × 10(-10) mol cm(-2)) ~ reductive annealed pretreatment (7.8 × 10(-10) mol cm(-2)) > UV/O(3) pretreatment (5.0 × 10(-10) mol cm(-2)) ~ piranha reagents pretreatment (4.1 × 10(-10) mol cm(-2)) ~ simple polishing pretreatment (4.0 × 10(-10) mol cm(-2)). This indicates that Au surfaces pretreated by aqua regia and reductive annealing can achieve the best results, and the Γ(m) values obtained are consistent with the theoretical coverage values (Γ(m) ≈ 8.0 × 10(-10) mol cm(-2)); however, the Γ(m) values for other three pretreatment methods (UV/O(3), piranha reagents and simple polishing) are not satisfactory, obtaining only almost half of the theoretical Γ(m) value. Thus, we recommend aqua regia and reductive annealed pretreatments as the best methods for self-assembling the alkyl thiol monolayer (C(n)SH-SAMs-Au), whereas UV/O(3), piranha reagents and simple polishing pretreatments are not recommended.

Exploration of the specific structural characteristics of thiol-modified single-stranded DNA self-assembled monolayers on gold by a simple model.

This paper proposed a simple hexagonal model to explore the specific structural characteristics of thiol-modified single-stranded DNA (ss-DNA) self-assembled monolayers (SAMs) on gold substrate. The calibrated gyration diameter d'(g)(d'(g)=rd(g)) was used to quantify the size of ss-DNA molecules on gold by introducing a calibrating factor r, where d(g) was ss-DNA gyration diameter in solution. Based on the model, the interfacial parameters of ss-DNA-SAMs on gold assembled under different ionic strength were obtained theoretically. The ss-DNA-SAMs were assembled on gold under different concentrations of C(NaCl) and six important electrochemical parameters were used to validate the model experimentally, which include surface coverage (Γ(m)), interfacial capacitance (C), phase angle (Ф(1 Hz)), ions transfer resistance (R(it)(*)), current density difference (Δj) and charge transfer resistance (R(ct)) from chronocoulometry (CC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Three main aspects were included in this paper: (1) construction of a simple hexagonal model to describe the specific structural characteristics of ss-DNA-SAMs on gold; (2) calculation of the calibrating factor r by CC experiments and several important conclusions from the simple model; and (3) confirmation of the simple model by our experimental results and literature reports. The simple model may provide an important reference for optimizing the design of DNA sensor.

Electrochemical studies on the permeable characteristics of thiol-modified double-stranded DNA self-assembled monolayers on gold.

In this article we studied the permeable characteristics of thiol-modified double-stranded DNA (ds-DNA) self-assembled monolayers (SAMs) on a gold substrate assembled under different NaCl concentrations by electrochemical methods. It was based on the inspection of five important parameters including interfacial capacitance (C), phase angle (Φ(1 Hz)), ions transfer resistance (R(it)*), current density difference (Δj) and electron transfer rate (k(et)) through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Three sections were included: (1) Investigation of the relationships of C, Φ(1 Hz), R(it)*, Δj and k(et) with NaCl concentrations and comparison with the reports from literature. Experimental results showed that ds-DNA-SAMs were permeable films. (2) Construction of a simple model for exploring the permeable characteristics of ds-DNA-SAMs on gold. (3) Confirmation of the simple model by chronocoulometry (CC) and application of the model to explain the permeable mechanism. This study was significant for exploring the mechanism of electron transfer through the interior of ds-DNA duplex helix.

Gold nanoparticle-enhanced chemiluminescence detection for CE.

Introducing gold nanoparticles (AuNPs) to the running buffer further improved the sensitivity of luminol-H(2)O(2) chemiluminescence (CL) detection for CE. This has led to the development of sensitive CE-CL assays of biomedically interesting compounds. Epinephrine and norfloxacin were taken as the model analytes. Epinephrine inhibited light emission from the AuNP-catalyzed CL while norfloxacin enhanced it. The CE-CL assays had detection limits of 6.9x10(-9) M for epinephrine and 7.3x10(-9) M for norfloxacin. It was noted that in the absence of AuNPs no CE-CL analytical signal was produced by epinephrine at 4.0x10(-5) M or norfloxacin at 1.5x10(-3) M under similar experimental conditions. Fluorescence spectroscopic measurements showed that although the fluorescence excitation/emission maxima remained the same, the fluorescence lifetime of luminol increased significantly in the presence of AuNPs (tau(2) increased from 8.49+/-0.12 to 9.18+/-0.047 ns in a two-exponential fit), indicating that the excited states of luminol molecules were stabilized through the interaction between luminol molecules and AuNPs. Finally, quantitation of epinephrine and norfloxacin in biological samples such as human urine by using the present AuNP-enhanced CE-CL method was demonstrated.