Self-assembled monolayers (SAMs) for electrochemical sensing

The past, present, and future of the application of self-assembled monolayers (SAMs) in electroanalytical chemistry is reviewed. SAMs for electroanalytical applications have been introduced in the early 1990s and since then have been exploited for the detection of different species ranging from metal ions to biomolecules and microorganisms. This review describes the different types of monolayers, surfaces on which they have been assembled, the various analytes, which were determined, and the various electrochemical techniques employed. The prospective and perspectives of this topic are discussed.     

Study on surface acid-base property of carboxylic acid-terminated self-assembled monolayers by cyclic voltammetry and electrochemical impedance spectroscopy

Cyclic voltammetry and electrochemical impedance spec-troscopy were used to study the surface acid-base property of carboxylic acid-terminated self-assembled monolayers(SAMs).A carboxylic acid-terminated thiol,such as thioctic acid(1,2-dithiolane-3-pentanoic acid),was self-assembled on gold electrodes.Electron transfer between the bulk solution and the SAM modified electrode was studied at different pH using Fe(CN)63-as a probe.The surface pka of thioctic acid was determined by cyclic voltammetry and electrochemical impedance spectroscopy to be 5.6 ±0.1 and 5.8±0.1,respectively.The method is compared with other methods of monolayer pKa measurement.     

Self-assembled monolayers (SAMs) of carboxylic acids: an overview

The field of self-assembled monolayers (SAMs) of organic compounds on different substrates is of importance because it provides a suitable and efficient method of surface modification. The formation of robust, stable monolayers from carboxylic acids on two and three dimensional surfaces of different substrates have been reported. Carboxylic acids are promising class of organic compounds for monolayer formations where traditional alkanethiols or alkoxysilanes show limitations.     

Characterization of gold-thiol-8-hydroxyquinoline self-assembled monolayers for selective recognition of aluminum ion using voltammetry and electrochemical impedance spectroscopy

Gold electrode surface is modified via covalent attachment of a synthesized thiol functionalized with 8-hydroxyquinoline, p-((8-hydroxyquinoline)azo) benzenethiol (SHQ), for the first time. The behavior of the nanostructured electrode surface (Au–SHQ) is characterized by electrochemical techniques including cyclic and differential pulse voltammetry (CV and DPV), and electrochemical impedance spectroscopy (EIS). The modified surface is stable in a wide range of potentials and pHs. A surface pK_a of 6.0 ± 0.1 is obtained for Au–SHQ electrode using surface acid/base titration curves constructed by CV and EIS measurements as a function of pH. These results helped to determine the charge state of the surface as a function of pH. The gold modified electrode surface showed good affinity for sensing the Al(III) ion at pH 5.5. The sensing process is based on (i) accumulation and complex formation between Al(III) from the solution phase and 8HQ function on the Au electrode surface (recognition step) and (ii) monitoring the impedance of the Au–SHQ–Al(III) complex against redox reaction rate of parabenzoquinone (PBQ) (signal transduction step). The PBQ is found to be a more suitable probe for this purpose, after testing several others. Thus, the sensor was tested for quantitative determination of Al(III) from the solution phase. At the optimized conditions, a linear response, from 1.0 × 10⁻¹¹ to 1.2 × 10⁻⁵ M Al(III) in semi-logarithmic scale, with a detection limit of 8.32 × 10⁻¹² M and mean relative standard deviation of 3.2% for n = 3 at 1.0 × 10⁻⁷ M Al(III) is obtained. Possible interferences from coexisting cations and anions are also studied. The results show that many ions do not interfere significantly with the sensor response for Al(III). Validity of the method and applicability of the sensor are successfully tested by determination of Al(III) in human blood serum samples.     

Application of electrochemical impedance spectroscopy to the study of dioleoyl phosphatidylcholine monolayers on mercury

Electrochemical impedance spectroscopy has been applied to the analysis of the behavior of monolayers of dioleoyl phosphatidylcholine (DOPC) on a mercury electrode. Experiments were carried out in electrolytes KCl and NaCl (0.1 mol dm(-3)) and Mg(NO3)2 (0.05 mol dm(-3)), and the frequency dependence of the complex impedance was estimated between 65 000 and 0.1 Hz at potentials -0.4 to -1.5 V versus Ag/AgCl 3.5 mol dm(-3) KCl at uncoated and coated electrode surfaces. Experiments were also carried out in the presence of gramicidin A (gA). Between the potentials of -0.4 and -0.7 V, the DOPC monolayer behaves as an almost ideal capacitor with little frequency dispersion. At more negative potentials, the impedance data show the formation of defects (-0.7 to -0.85 V), ingression of electrolyte into the layer (capacitance peak approximately -0.935 V), reorientation of phospholipid-water structures (capacitance peak approximately -1.0 V), and initiation of phospholipid desorption (approximately -1.3 V). gA interaction with the phospholipid monolayer at -0.4 V is shown as an additional low-frequency element. A general "one capacitor model" in a RC series equivalent circuit is developed incorporating the frequency dispersion of the capacitance, distribution of the time constants of the dispersion, and a coefficient related to the interface between the solution and the coated electrode. This latter coefficient is the most robust and decreases at potentials approaching those coincident with the DOPC phase transitions.     

Label-free monitoring of site-specific DNA cleavage by EcoRI endonuclease using cyclic voltammetry and electrochemical impedance

Cyclic voltammetry (CV) combined with electrochemical impedance spectroscopy (EIS) were proposed to monitor the site-specific DNA cleavage by EcoRI endonuclease without using external label. The alteration of CV and EIS signal demonstrated that double-strands (dsDNA) contain recognition sequence was cleaved by EcoRI endonuclease. Real-time monitoring indicated that the dsDNA was cleaved by EcoRI more than 90% after 2 h of enzyme digestion time. Control experiment showed that the DNA cleavage by EcoRI endonuclease is site-specific for DNA sequence. Experimental results demonstrated that the efficiency of EcoRI cleavage was highly dependent on the concentration of EcoRI concentration in the range from 0.04 to 0.4 U μL⁻¹ with one almost linear relationship.     

Heterogeneous electron transfer kinetics at the ionic liquid/metal interface studied using cyclic voltammetry and scanning electrochemical microscopy

The electrochemical behavior of a redox-active, ferrocene-modified ionic liquid (1-ferrocenylmethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) in acetonitrile and in an ionic liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is reported. Reversible electrochemical behavior was observed in each electrolyte with responses typical of those for unmodified ferrocene observed in each medium. In the ionic liquid electrolyte, the diffusion coefficient of the redox-active ionic liquid increased by a factor of 5 upon increasing the temperature from 27 to 90 degrees C. The kinetics of electron transfer across the ionic liquid/electrode interface were studied using cyclic voltammetry, and the standard heterogeneous electron transfer rate constant, k (0) was determined to be 4.25 x 10 (-3) cm s (-1). Scanning electrochemical microscopy was then also used to probe the heterogeneous kinetics at the interface between the ionic liquid and the solid electrode and conventional kinetic SECM theory was used to determine k (0). The k (0) value obtained using SECM was higher than that determined using cyclic voltammetry. These results indicate that SECM is a very useful technique for studying electron transfer dynamics in ionic liquids.     

Characterization of carboxylic acid-terminated self-assembled monolayers by electrochemical impedance spectroscopy and scanning electrochemical microscopy

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.     

Nanostructured platform for the detection of Neisseria gonorrhoeae using electrochemical impedance spectroscopy and differential pulse voltammetry

We report on a nanocomposite based genosensor for the detection of Neisseria gonorrhoeae, a bacterium causing the sexually transmitted disease gonorrhoea. Amino-labeled probe DNA was covalently immobilized on electrochemically prepared polyaniline and iron oxide (PANI-Fe₃O₄) nanocomposite film on an indium tin oxide (ITO) electrode. Scanning electron microscopy, transmission electron microscopy, electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) techniques have been employed to characterize surface of the modified electrode. The genosensor has detection limits of 1?×?10^-15 M and 1?×?10^-17 M, respectively, using the EIS and DPV techniques. This biosensor can discriminate a complementary sequence from a single-base mismatch and from non-complementary DNA, and has been utilized for detection of DNA extracted from N. gonorrhoeae culture, and from patient samples with N. gonorrhoeae. It is found to exhibit good specificity for N. gonorrhoeae species and shows no response towards non-gonorrhoeae type of Neisseria species (NgNs) and other gram-negative bacterias (GNBs). The affinity constant for hybridization calculated using the Langmuir adsorption isotherm model is found to be 3.39?×?10⁸ M^-1.

The electrochemical behaviour of butyltrimethylammonium bis(trifluoromethylsulfonyl)imide at negatively polarised aluminium electrode studied by in situ soft X-ray photoelectron spectroscopy,electrochemical impedance spectroscopy and cyclic voltammetry techniques

The in situ X-ray photoelectron spectroscopy data indicate that butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N4111(TFSI)) adsorbs strongly within the potential range −3.25 V < E < −2.25 V and specifically at E < −3.25 V (vs. Ag-QRE) at the Al electrode. Strong adsorption of the intermediates of N4111(TFSI) electrochemical decomposition was observed in electrochemical impedance spectroscopy and cyclic voltammetry measurements. At E < −4.25 V (vs. Ag-QRE), very intensive electrochemical reduction of N4111(TFSI) took place at the Al electrode giving gaseous products. In the potential range from − 2.25 to 0.00 V (vs. Ag-QRE), non-specific adsorption of N4111(TFSI) exists et al. surface.

     

Study of the adsorption of cytochrome c on a gold nanoparticle – modified gold electrode by using cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry

Adsorption of cytochrome c (Cyt c) on a gold nanoparticle – modified gold electrode was studied by using cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry in a phosphate buffer solution. It is shown that the charge transfer resistance is directly proportional to the amount of adsorbed Cyt c. The effects of temperature and time on the course of adsorption were also studied. The trends obtained in ΔG_ADS showed that Cyt c was found to have a smaller affinity for the modified electrode as indicated by their smaller negative ΔG_ADS values.     

Studying electron transfer reaction at the Au/n-decanethiol/aqueous solution of NaNO3 interface by electrochemical impedance spectroscopy

The electrochemical behavior of the gold/electrolyte interface in aqueous 1 M NaNO₃ solutions in the presence of an organic monolayer of n-decanethiol (CH₃(CH₂)₉S) is studied by electrochemical impedance spectroscopy in the frequency range of 10–10⁵ Hz and also by cyclic voltammetry. It is experimentally shown that in the potential interval from 0 to ?0.5 V (vs. SCE), the dense monolayer film decreases the measured current density approximately 40-fold. The measured capacitance falls down to 1–2 μF/cm². Based on the analysis of impedance characteristics acquired with the use of empirical equivalent circuits comprising ideal and nonideal analogues of electric circuits, the tentative estimates of the thickness of organic monolayers formed on Au electrodes with various roughness factors are obtained. Using the complex nonlinear regression (CNLS) method and a model of microarray electrode, the porous structure of adsorbed monolayers is revealed and the transition frequency of interfaces under study is determined. The degree of inhibition of the electron transfer across the Au/n-decanethiol/solution interface is determined by comparing the rate constants for the Ru[(NH₃)₆]^3+/2+ redox process on clean and modified electrodes. The acquired results are compared with available literature data.     

Application of the voltammetry of microparticles for dating archaeological lead using polarization curves and electrochemical impedance spectroscopy

The application of the voltammetry of microparticles methodology to date archaeological lead artifacts, based on the time-dependent formation of different layers of lead oxides, whose relative amount can be estimated from polarization curves and electrochemical impedance spectroscopy (EIS), is presented. This approach is complemented by additional data using square wave voltammetry data. Calibration of the method was performed with the help of a series of well-documented, lead pieces from the funds of different Spanish museums, covering since the 7th century BC to nowadays.     

Electrochemical behaviour of carbon paste electrodes enriched with tin oxide nanoparticles using voltammetry and electrochemical impedance spectroscopy

The effect of the SnO(2) nanoparticles (SNPs) on the behaviour of voltammetric carbon paste electrodes were studied for possible use of this material in biosensor development. The electrochemical behaviour of SNP modified carbon paste electrodes (CPE) was first investigated by using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) techniques. The performance of the SNP modified electrodes were compared to those of unmodified ones and the parameters affecting the response of the modified electrode were optimized. The SNP modified electrodes were then tested for the electrochemical sensing of DNA purine base adenine to explore their further development in biosensor applications.     

Redox capacitance of poly-3,4-ethylenedioxythiophene studied by cyclic voltammetry and faradaic impedance spectroscopy

The paper presents results of studying the electrochemical properties of poly-3,4-ethylenedioxythiophene films using the methods of cyclic voltammetry and faradaic impedance spectroscopy in acetonitrile and propylene carbonate solutions of different electrolytes: LiClO₄, Bu₄NBF₄, Bu₄NPF₆. The effect of the film synthesis conditions, the nature of anion and solvent, and the supporting electrolyte concentration on the film redox capacitance is discussed. Main attention is paid to the comparison of values characterizing the redox capacitance of a poly-3,4-ethylenedioxythiophene film C _lf (determined using the faradaic impedance method) and C _cv (determined using the cyclic voltammetry method) and the studies of their dependence on the varied experimental factors. The experimental C _lf vs. E curves are analyzed using the relationships of a model of a uniform film.     

Insights into electrocatalytic activity of epitaxial graphene on SiC from cyclic voltammetry and ac impedance spectroscopy

Electrocatalytic activity of graphene grown epitaxially on SiC is studied using cyclic voltammetry and electrochemical impedance spectroscopy. AFM images show step-like topography of SiC-graphene. For ferri-/ferrocyanide redox couple, no voltammetric response is observed at the pristine graphene. Basal planes of graphite are electrochemically inactive as well. After electrochemical oxidation, apparent redox peaks appear at both the graphene and graphite electrode. However, more intensive redox peaks are observed at graphene, where simultaneous redox reaction with the adsorbed and the diffused ferri-/ferrocyanide ions occurs. Electrochemical impedance measurements show that the graphene electrode behaves like an array of microelectrodes. We used the partially blocked electrode model to fit impedance data. Using the fitting parameters, a size of microelectrodes was found to be 23.8?±?2.1 μm and the active surface of graphene was estimated to be 21 %. A value of the standard electron transfer rate constant found for the anodized epitaxial graphene (2.16?±?0.32)?×?10^??3cm???s^??1) is by one order of magnitude lower than the standard rate constant estimated for the anodized graphite basal planes (～5?×?10^??2cm???s^??1). Electrochemical reduction causes total disappearance of electrochemical responses at the graphene electrode, whereas only slight decrease of the peak currents is observed at the reduced graphene. Such behavior proves that different activation mechanisms occur at the graphene and graphite electrodes.     

Self-assembled monolayers of alkaneselenolates on (111) gold and silver

Self-assembled monolayers (SAMs) formed from didodecyl diselenide (C12SeSeC12) and didodecyl selenide (C12SeC12) on (111) Au and Ag substrates were extensively characterized by several complementary techniques. C12SeSeC12 was found to form contamination-free, densely packed, and well-ordered C12Se SAMs on both substrates, whereas the adsorption of C12SeC12 occurred only on Au and resulted in the formation of a SAM-like C12SeC12 film with a low packing density and a conformational disorder. The properties of the C12Se SAMs were compared with those of dodecanethiolate (C12S) SAMs. The packing density, orientational order, and molecular inclination in C12Se/Au and C12S/Au were found to be very similar. In contrast, C12Se/Ag exhibited significantly lower packing density, a lower degree of the conformational and orientational order, and a larger molecular inclination than C12S/Ag. The results suggest a sp3 bonding configuration for the selenium atom on Au and Ag and indicate a larger corrugation of the headgroup-substrate binding energy surface in C12Se/Ag than in C12S/Ag.     

Dendrimer modified graphite sensors for detection of anticancer drug Daunorubicin by voltammetry and electrochemical impedance spectroscopy

The development of amino-terminated G4 PAMAM dendrimer (PDR) modified disposable electrodes were developed as the first time in our study by using the dendrimer modified disposable graphite (PDR-PGE) and multiwalled carbon nanotube based screen-printed graphite (PDR-MWCNT-SPE) electrodes. Firstly, the microscopic characterization of bare PGEs and PDR modified PGEs was performed. These sensors were then applied for electrochemical monitoring of an anticancer drug, Daunorubicin (DNR). The enhanced oxidation signal of DNR was measured at +0.50 V by using differential pulse voltammetry (DPV) in combination with the PDR-PGEs. The detection limit, estimated from S/N = 3, corresponds accordingly to 317 nM and 128 nM for DNR respectively at the PGE and PDR-PGE. The voltammetric results were consistent with electrochemical impedance spectroscopy (EIS) that was used to characterize the successful modification of PDR onto the surface of PGE and MWCNT-SPE.     

UV photooxidation and photopatterning of alkanethiolate self-assembled monolayers (SAMs) on GaAs (001)

We have investigated the photooxidation of alkanethiolate self-assembled monoalyers (SAMs) adsorbed on GaAs (001) using time-of-flight secondary ion mass spectrometry. Both -CH3- and -COOH-terminated SAMs undergo photoreaction to form sulfonated species upon exposure to UV light from a 500 W Hg arc lamp (lambda = 280-440 nm) in the presence of oxygen. In contrast to SAMs adsorbed on metals, the photooxidation of octadecanethiol adsorbed on GaAs can be fit to two first-order reactions: a fast initial reaction followed by a second slower reaction ( approximately 6 times slower). For SAMs with shorter alkyl chain lengths, the photooxidation process is can be fit to a single first-order reaction. Using the optimal photooxidation time, we also demonstrate that SAMs can be successfully UV photopatterned on GaAs substrates producing sharp, well-defined patterns.