Frequency-selective electrokinetic enrichment of biomolecules in physiological media based on electrical double-layer polarization.

Proteomic biomarkers of interest to the early diagnosis of diseases and infections are present at trace levels versus interfering species. Hence, their selective enrichment is needed within bio-assays for speeding binding kinetics with receptors and for reducing signal interferences. While DC fields can separate biomolecules based on their electrokinetic mobilities, they are unable to selectively enrich biomarkers versus interfering species, which may possess like-charges. We present the utilization of AC electrokinetics to enable frequency-selective enrichment of nanocolloidal biomolecules, based on the characteristic time constant for polarization of their electrical double-layer, since surface conduction in their ion cloud depends on colloidal size, shape and surface charge. In this manner, using DC-offset AC fields, differences in frequency dispersion for negative dielectrophoresis are balanced against electrophoresis in a nanoslit channel to enable the selective enrichment of prostate specific antigen (PSA) versus anti-mouse immunoglobulin antibodies that cause signal interferences to immunoassays. Through coupling enrichment to capture by receptors on graphene-modified surfaces, we demonstrate the elimination of false positives caused by anti-mouse immunoglobulin antibodies to the PSA immunoassay.

Aptamer-functionalized nanoparticles for surface immobilization-free electrochemical detection of cortisol in a microfluidic device.

Monitoring the periodic diurnal variations in cortisol from small volume samples of serum or saliva is of great interest, due to the regulatory role of cortisol within various physiological functions and stress symptoms. Current detection assays are immunologically based and require cumbersome antibody immobilization chemistries, thereby limiting the assay versatility, kinetics, and reproducibility. We present a quantitative aptamer-based detection methodology for cortisol that does not require target labeling, capture probe immobilization on the detection surface or wash steps prior to readout. Using a recognition system of aptamer functionalized gold nanoparticles pre-bound with electro-active triamcinolone, the cortisol level is detected based on its competitive binding to the aptamer by following signal from the displaced triamcinolone using square wave voltammetry at patterned graphene-modified electrodes in a microfluidic or nanoslit device. Due to the 3D analyte diffusion profile at the aptamer interface and the ability to enhance the surface area for cortisol capture, this assay shows signal linearity over a five-log analyte concentration range (10 µg/mL to 30 pg/mL) and exhibits rapid binding kinetics with cortisol versus other glucocorticoids, as apparent from the absence of interferences from estradiol, testosterone and progesterone. The assay is carried out within the biologically relevant range for glucocorticoids in serum and saliva matrices, and benchmarked versus ELISA and radioimmunoassays. Based on absence of cumbersome surface immobilization and wash steps for carrying out this assay, its quantitative signal characteristics and its ability to resist interferences from other glucocorticoids, we envision its application towards routine monitoring of cortisol within bio-fluids.

Correction: Ultrafast immunoassays by coupling dielectrophoretic biomarker enrichment in nanoslit channel with electrochemical detection on graphene.

Correction for 'Ultrafast immunoassays by coupling dielectrophoretic biomarker enrichment in nanoslit channel with electrochemical detection on graphene' by Bankim J. Sanghavi et al., Lab Chip, 2015, DOI: 10.1039/c5lc00840a.

Ultrafast immunoassays by coupling dielectrophoretic biomarker enrichment in nanoslit channel with electrochemical detection on graphene.

Heterogeneous immunoassays usually require long incubation times to promote specific target binding and several wash steps to eliminate non-specific binding. Hence, signal saturation is rarely achieved at detection limit levels of analyte, leading to significant errors in analyte quantification due to extreme sensitivity of the signals to incubation time and methodology. The poor binding kinetics of immunoassays at detection limit levels can be alleviated through creating an enriched analyte plug in the vicinity of immobilized capture probes to enable signal saturation at higher levels and at earlier times, due to higher analyte association and its faster replenishment at the binding surface. Herein, we achieve this by coupling frequency-selective dielectrophoretic molecular dam enrichment of the target biomarker in physiological media to capture probes immobilized on graphene-modified surfaces in a nanoslit to enable ultrafast immunoassays with near-instantaneous (<2 minutes) signal saturation at dilute biomarker levels (picomolar) within ultra-low sample volumes (picoliters). This methodology is applied to the detection of Prostate Specific Antigen (PSA) diluted in serum samples, followed by validation against a standard two-step immunoassay using three de-identified patient samples. Based on the ability of dielectrophoretic molecular dam analyte enrichment methods to enable the detection of PSA at 1-5 pg mL(-1) levels within a minute, and the relative insensitivity of the signals to incubation time after the first two minutes, we envision its application for improving the sensitivity of immunoassays and their accuracy at detection limit levels.

Nanomaterial-based electrochemical sensing of neurological drugs and neurotransmitters.

Nanomaterial-modified detection systems represent a chief driver towards the adoption of electrochemical methods, since nanomaterials enable functional tunability, ability to self-assemble, and novel electrical, optical and catalytic properties that emerge at this scale. This results in tremendous gains in terms of sensitivity, selectivity and versatility. We review the electrochemical methods and mechanisms that may be applied to the detection of neurological drugs. We focus on understanding how specific nano-sized modifiers may be applied to influence the electron transfer event to result in gains in sensitivity, selectivity and versatility of the detection system. This critical review is structured on the basis of the Anatomical Therapeutic Chemical (ATC) Classification System, specifically ATC Code N (neurotransmitters). Specific sections are dedicated to the widely used electrodes based on the carbon materials, supporting electrolytes, and on electrochemical detection paradigms for neurological drugs and neurotransmitters within the groups referred to as ATC codes N01 to N07. We finally discuss emerging trends and future challenges such as the development of strategies for simultaneous detection of multiple targets with high spatial and temporal resolutions, the integration of microfluidic strategies for selective and localized analyte pre-concentration, the real-time monitoring of neurotransmitter secretions from active cell cultures under electro- and chemotactic cues, aptamer-based biosensors, and the miniaturization of the sensing system for detection in small sample volumes and for enabling cost savings due to manufacturing scale-up. The Electronic Supporting Material (ESM) includes review articles dealing with the review topic in last 40 years, as well as key properties of the analytes, viz., pKa values, half-life of drugs and their electrochemical mechanisms. The ESM also defines analytical figures of merit of the drugs and neurotransmitters. The article contains 198 references in the main manuscript and 207 references in the Electronic Supporting Material. Figureᅟ

Electrokinetic preconcentration and detection of neuropeptides at patterned graphene-modified electrodes in a nanochannel.

Neuropeptides are vital to the transmission and modulation of neurological signals, with Neuropeptide Y (NPY) and Orexin A (OXA) offering diagnostic information on stress, depression, and neurotrauma. NPY is an especially significant biomarker, since it can be noninvasively collected from sweat, but its detection has been limited by poor sensitivity, long assay times, and the inability to scale-down sample volumes. Herein, we apply electrokinetic preconcentration of the neuropeptide onto patterned graphene-modified electrodes in a nanochannel by frequency-selective dielectrophoresis for 10 s or by electrochemical adsorptive accumulation for 300 s, to enable the electrochemical detection of NPY and OXA at picomolar levels from subnanoliter samples, with sufficient signal sensitivity to avoid interferences from high levels of dopamine and ascorbic acid within biological matrices. Given the high sensitivity of the methodology within small volume samples, we envision its utility toward off-line detection from droplets collected by microdialysis for the eventual measurement of neuropeptides at high spatial and temporal resolutions.

Voltammetric determination of sumatriptan based on a graphene/gold nanoparticles/Nafion composite modified glassy carbon electrode.

A mixture of graphene oxide and tetrachloroauric acid was electrochemically co-reduced directly on a glassy carbon electrode (GCE) surface via cyclic voltammetry so as to form a graphene (Gr)-gold nanoparticles (AuNP) composite. This nanocomposite was then coated with nafion (NAF) film so as to form Gr/AuNP/NAF/GCE. Sumatriptan (SUM) is a drug belonging to the triptan class, used for the treatment of migraine headaches. In this work, an electrochemical method based on the adsorptive stripping differential pulse voltammetry (AdSDPV) employing Gr/AuNP/NAF/GCE has been proposed for the subnanomolar determination of SUM. Characterization of the electrode material has been carried out by UV-visible spectrophotometry, X-ray diffraction and scanning electron microscopy. Also the electrode surface has been characterized by means of cyclic voltammetry, electrochemical impedance spectroscopy, chronocoulometry. By employing Gr/AuNP/NAF/GCE at pH 7.0 phosphate buffer, a 20-fold enhancement in the AdSDPV signal was observed as compared to GCE. Under the optimized conditions, Ip (µA) was proportional to the SUM concentration in the range of 1.0×10(-6)-4.12×10(-5) M (R(2)=0.9991) and 2.14×10(-9)-1.0×10(-6) M (R(2)=0.9954) with a detection limit (3×SD/s) of 7.03×10(-10) M. The practical analytical utilities of the modified electrode were demonstrated by the determination of SUM in pharmaceutical formulations, human urine and blood serum samples. This proposed method was validated by HPLC and the results are in agreement at the 95% confidence level.

Real-time electrochemical monitoring of adenosine triphosphate in the picomolar to micromolar range using graphene-modified electrodes.

We report on a competitive electrochemical detection system that is free of wash steps and enables the real-time monitoring of adenosine triphosphate (ATP) in a quantitative manner over a five-log concentration range. The system utilizes a recognition surface based on ATP aptamer (ATPA) capture probes prebound to electroactive flavin adenine dinucleotide (FAD) molecules, and a signaling surface utilizing graphene (Gr) and gold nanoparticle (AuNP) modified carbon paste electrode (Gr-AuNP-CPE) that is optimized to enhance electron-transfer kinetics and signal sensitivity. Binding of ATP to ATPA at the recognition surface causes the release of an equivalent concentration of FAD that can be quantitatively monitored in real time at the signaling surface, thereby enabling a wide linear working range (1.14 × 10(-10) to 3.0 × 10(-5) M), a low detection limit (2.01 × 10(-11) M using graphene and AuNP modified glassy carbon), and fast target binding kinetics (steady-state signal within 12 min at detection limit). Unlike assays based on capture probe-immobilized electrodes, this double-surface competitive assay offers the ability to speed up target binding kinetics by increasing the capture probe concentration, with no limitations due to intermolecular Coulombic interactions and nonspecific binding. We utilize the real-time monitoring capability to compute kinetic parameters for target binding and to make quantitative distinctions on degree of base-pair mismatch through monitoring target binding kinetics over a wide concentration range. On the basis of the simplicity of the assay chemistry and the quantitative detection of ATP within fruit and serum media, as demonstrated by comparison of ATP levels against those determined using a standard high-performance liquid chromatography (HPLC)-UV absorbance method, we envision a versatile detection platform for applications requiring real-time monitoring over a wide target concentration range.

Adsorptive stripping voltammetric determination of imipramine, trimipramine and desipramine employing titanium dioxide nanoparticles and an Amberlite XAD-2 modified glassy carbon paste electrode.

An Amberlite XAD-2 (XAD2) and titanium dioxide nanoparticles (TNPs) modified glassy carbon paste electrode (XAD2-TNP-GCPE) was developed for the determination of imipramine (IMI), trimipramine (TRI) and desipramine (DES). The electrochemical behavior of these molecules was investigated employing cyclic voltammetry (CV), chronocoulometry (CC), electrochemical impedance spectroscopy (EIS) and adsorptive stripping differential pulse voltammetry (AdSDPV). After optimization of analytical conditions using a XAD2-TNP-GCPE electrode at pH 6.0 phosphate buffer (0.1 M), the peak currents were found to vary linearly with its concentration in the range of 1.30 × 10(-9) to 6.23 × 10(-6) M for IMI, 1.16 × 10(-9) to 6.87 × 10(-6) M for TRI and 1.43 × 10(-9) to 5.68 × 10(-6) M for DES. The detection limits (S/N = 3) of 3.93 × 10(-10), 3.51 × 10(-10) and 4.35 × 10(-10) M were obtained for IMI, TRI and DES respectively using AdSDPV. The prepared modified electrode showed several advantages such as a simple preparation method, high sensitivity, very low detection limits and excellent reproducibility. The proposed method was employed for the determination of IMI, TRI and DES in pharmaceutical formulations, blood serum and urine samples.

Biomimetic sensor for certain catecholamines employing copper(II) complex and silver nanoparticle modified glassy carbon paste electrode.

A dimeric Cu(II) complex [Cu(µ(2)-hep)(hep-H)](2)·2ClO(4) (1) containing bidentate (hep-H=2-(2-hydroxyethyl)pyridine) ligand was synthesized and characterized by single crystal X-ray diffraction studies. Each Cu-ion in 1 is in a distorted square pyramidal geometry. Further 1 along with silver nanoparticles (SNPs) have been used as modifier in the construction of a biomimetic sensor (1-SNP-GCPE) for determining certain catecholamines viz., dopamine (DA), levodopa (l-Dopa), epinephrine (EP) and norepinephrine (NE) using cyclic voltammetry, chronocoulometry, electrochemical impedance spectroscopy and adsorptive stripping square wave voltammetry (AdSSWV). Finally, the catalytic properties of the sensor were characterized by chronoamperometry. Employing AdSSWV, the calibration curves showed linear response ranging between 10(-6) and 10(-9)M for all the four analytes with detection limits (S/N=3) of 8.52×10(-10)M, 2.41×10(-9)M, 3.96×10(-10)M and 3.54×10(-10)M for DA, l-Dopa, EP and NE respectively. The lifetime of the biomimetic sensor was 3 months at room temperature. The prepared modified electrode shows several advantages such as simple preparation method, high sensitivity, high stability, ease of preparation and regeneration of the electrode surface by simple polishing along with excellent reproducibility. The method has been applied for the selective and precise analysis of DA, l-Dopa, EP and NE in pharmaceutical formulations, urine and blood serum samples.

Potentiometric stripping analysis of methyl and ethyl parathion employing carbon nanoparticles and halloysite nanoclay modified carbon paste electrode.

Carbon nanoparticles (CNPs) and halloysite nanoclay (HNC) modified carbon paste electrode (HNC-CNP-CPE) was developed for the determination of methyl parathion (MP) and ethyl parathion (EP). The electrochemical behavior of these molecules was investigated employing cyclic voltammetry (CV), chronocoulometry (CC), electrochemical impedance spectroscopy (EIS) and potentiometric stripping analysis (PSA). After optimization of analytical conditions employing this electrode at pH 5.0 in acetate buffer (0.1 M), the peak currents were found to vary linearly with its concentration in the range of 1.55×10(-9) to 3.67×10(-6) M and 1.21×10(-9) to 4.92×10(-6) M for MP and EP, respectively. The detection limits (S/N=3) of 4.70×10(-10) M and 3.67×10(-10) M were obtained for MP and EP, respectively, using PSA. The prepared modified electrode showed several advantages such as simple preparation method, high sensitivity, very low detection limits and excellent reproducibility. The proposed method was employed for the determination of MP and EP in fruits, vegetables, water and soil samples.

Simultaneous voltammetric determination of acetaminophen and tramadol using Dowex50wx2 and gold nanoparticles modified glassy carbon paste electrode.

A glassy carbon paste electrode (GCPE) modified with a cation exchanger resin, Dowex50wx2 and gold nanoparticles (D50wx2-GNP-GCPE) has been developed for individual and simultaneous determination of acetaminophen (ACOP) and tramadol (TRA). The electrochemical behavior of both the molecules has been investigated employing cyclic voltammetry (CV), chronocoulometry (CC), electrochemical impedance spectroscopy (EIS) and adsorptive stripping square wave voltammetry (AdSSWV). The studies revealed that the oxidation of ACOP and TRA is facilitated at D50wx2-GNP-GCPE. Using AdSSWV, the method allowed simultaneous determination of ACOP and TRA in the linear working range of 3.34×10(-8) to 4.22×10(-5) M with detection limits of 4.71×10(-9) and 1.12×10(-8) M (S/N=3) for ACOP and TRA respectively. The prepared modified electrode shows several advantages such as simple preparation method, long-time stability, ease of preparation and regeneration of the electrode surface by simple polishing and excellent reproducibility. The high sensitivity and selectivity of D50wx2-GNP-GCPE were demonstrated by its practical application in the determination of both ACOP and TRA in pharmaceutical formulations, urine and blood serum samples.

Potentiometric stripping analysis of antimony based on carbon paste electrode modified with hexathia crown ether and rice husk.

An electrochemical method based on potentiometric stripping analysis (PSA) employing a hexathia 18C6 (HT18C6) and rice husk (RH) modified carbon paste electrode (HT18C6-RH-CPE) has been proposed for the subnanomolar determination of antimony. The characterization of the electrode surface has been carried out by means of scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy and chronocoulometry. By employing HT18C6-RH-CPE, a 12-fold enhancement in the PSA signal (dt/dE) was observed as compared to plain carbon paste electrode (PCPE). Under the optimized conditions, dt/dE (sV(-1)) was proportional to the Sb(III) concentration in the range of 1.42×10(-8) to 6.89×10(-11)M (r=0.9944) with the detection limit (S/N=3) of 2.11×10(-11)M. The practical analytical utilities of the modified electrode were demonstrated by the determination of antimony in pharmaceutical formulations, human hair, sea water, urine and blood serum samples. The prepared modified electrode showed several advantages, such as simple preparation method, high sensitivity, very low detection limit and excellent reproducibility. Moreover, the results obtained for antimony analysis in commercial and real samples using HT18C6-RH-CPE and those obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES) are in agreement at the 95% confidence level.

Biomimetic sensor for certain phenols employing a copper(II) complex.

A new dimeric Cu(II) complex [Cu(mu(2)-hep)(hep-H)](2).2PF(6) (1) containing a bidentate (hep-H = 2-(2-hydroxyethyl)pyridine) ligand was synthesized and characterized by single crystal X-ray diffraction studies. Each Cu ion in 1 is in a distorted square pyramidal geometry. Further 1 is used as a modifier in the construction of a biomimetic sensor for determining phenols [phenol (Phe), resorcinol (Res), hydroquinone (HQ), and catechol (Cat)] in phosphate buffer by using cyclic voltammetry (CV), chronocoulometry, electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV), and square wave voltammetry (SWV). DPV has been proposed for trace determination of Phe and Res while SWV for HQ and Cat. The method has been applied for the selective and precise analysis of Phe in commercial injections, Res in hair coloring agents, HQ in photographic developers and cosmetics, and Cat in tea samples and guarana tablets. The calibration curves showed a linear response ranging between 10(-6) and 10(-8) M for all four of the analytes with detection limits (3sigma) of 1.04 x 10(-8), 2.31 x 10(-8), 1.54 x 10(-8), and 0.86 x 10(-8) M for Phe, Res, HQ, and Cat, respectively. The lifetime of the biomimetic sensor was 200 days at room temperature (at least 750 determinations). The catalytic properties of 1-CPE were characterized by chronoamperometry and were found to be in good agreement with Michaelis-Menten kinetics.