Graphene ceramic composite as a new kind of surface-renewable electrode: application to the electroanalysis of ascorbic acid

This study introduces a new surface-renewable electrode based on a sol–gel derived graphene ceramic composite. The electrode was prepared by dispersing graphene nanosheets into a solution of the sol–gel precursors containing methyl triethoxysilane in methanol and hydrochloric acid. During hydrolysis of methyl triethoxysilane, the graphene nanosheets are trapped in the gel. After moulding and drying the composite, it can be used as a surface-renewable electrode to which we refer as a graphene ceramic composite electrode (GCCE). Cyclic voltammograms of the hexacyanoferrate(II/III) model redox system at the GCCE were compared to those obtained with a conventional carbon ceramic electrode and showed a highly improved electron transfer rate at the GCCE. The electrocatalytic oxidation of ascorbic acid as a model analyte was then studied at working potential of 50 mV and over the 3–84 μM concentration range. It revealed a sensitivity of 6.06 μA μM^?1 cm^?2 and a detection limit of 0.82 μM. The GCCE was successfully applied to the determination of ascorbic acid in orange juice and urine samples. Advantages such as good mechanical and chemical stability, ease of fabrication, and reproducible preparation make the GCCE a potentially useful and widely applicable renewable electrode for use in routine analysis. Fig. 1

(Left) FESEM image and photograph of the graphene ceramic composite electrode (GCCE); (right) the cyclic voltammogram of the renewable GCCE in 5 mM K₃[Fe(CN)₆] solution containing 0.1 M KNO₃ at scan rate of 100 mV s^?1      

Enzymatic glucose biosensor based on bismuth nanoribbons electrochemically deposited on reduced graphene oxide

We describe the electrochemical preparation of bismuth nanoribbons (Bi-NRs) with an average length of 100 ± 50 nm and a width of 10 ± 5 μm by a potentiostatic method. The process occurs on the surface of a glassy carbon electrode (GCE) in the presence of disodium ethylene diamine tetraacetate that acts as a scaffold for the growth of the Bi-NRs and also renders them more stable. The method was applied to the preparation of Bi-NRs incorporated into reduced graphene oxide. This nanocomposite was loaded with the enzyme glucose oxidase onto a glassy carbon electrode. The resulting biosensor displays an enhanced redox peak for the enzyme with a peak-to-peak separation of about 28 mV, revealing a fast electron transfer at the modified electrode. The loading of the GCE with electroactive GOx was calculated to be 8.54 × 10⁻¹⁰ mol∙cm⁻², and the electron transfer rate constant is 4.40 s⁻¹. Glucose can be determined (in the presence of oxygen) at a relatively working potential of −0.46 V (vs. Ag|AgCl) in the 0.5 to 6 mM concentration range, with a 104 μM lower detection limit. The sensor also displays appreciable repeatability, reproducibility and remarkable stability. It was successfully applied to the determination of glucose in human serum samples.

A potentiostatic method was used to prepare reduced graphene oxide and bismuth nanoribbons nanocomposite on a glassy carbon electrode. This nanocomposite was loaded with enzyme glucose oxidase to fabricate a glucose biosensor.

     

Titanium dioxide nanorod-based amperometric sensor for highly sensitive enzymatic detection of hydrogen peroxide

Titanium dioxide nanorods (TNR) were grown on a titanium electrode by a hydrothermal route and further employed as a supporting matrix for the immobilization of nafion-coated horseradish peroxidase (HRP). The strong electrostatic interaction between HRP and TNR favors the adsorption of HRP and facilitates direct electron transfer on the electrode. The electrocatalytic activity towards hydrogen peroxide (H₂O₂) was investigated via cyclic voltammetry and amperometry. The biosensor exhibits fast response, a high sensitivity (416.9 μA·mM⁻¹), a wide linear response range (2.5 nM to 0.46 mM), a detection limit as low as 12 nM, and a small apparent Michaelis-Menten constant (33.6 μM). The results indicate that this method is a promising technique for enzyme immobilization and for the fabrication of electrochemical biosensors.

A TiO₂ nanorod film was directly grown on Ti substrate by a hydrothermal route, and was further employed for a supporting matrix to immobilize horseradish peroxidase as a biosensor electrode. The as-prepared hydrogen peroxide biosensor based on Nafion/HRP/TNR/Ti electrode exhibited fast response and excellent electrocatalytic activity toward H₂O₂, i.e., a high sensitivity (416.9 μA mM⁻¹), a wide linear range (2.5 × 10⁻⁸ to 4.6 × 10⁻⁴ M) with a low detection limit (0.012 μM) and a small apparent Michaelis-Menten constant (33.6 μM).

     

Electrochemical immunoassay for the prostate specific antigen using a reduced graphene oxide functionalized with a high molecular-weight silk peptide

High molecular-weight silk peptide (SP) was used to functionalize the surface of nanosheets of reduced graphene oxide (rGO). The SP-rGO nanocomposite was then mixed with mouse anti-human prostate specific antigen monoclonal antibody (anti-PSA) and coated onto a glassy carbon electrode to fabricate an immunosensor. By using the hexacyanoferrate redox system as electroactive probe, the immunosensor was characterized by voltammetry and electrochemical impedance spectroscopy. The peak current, measured at the potential of 0.24 V (vs. SCE), is distinctly reduced after binding prostate specific antigen (PSA). Response (measured by differential pulse voltammetry) is linearly related to PSA concentration in the range from 0.1 to 5.0 ng · mL⁻¹ and from 5.0 to 80.0 ng∙mL⁻¹, and the detection limit is 53 pg∙mL⁻¹ (at an SNR of 3). The immunosensor was successfully applied to the determination of PSA in clinical serum samples, and the results were found to agree well with those obtained with an enzyme-linked immunosorbent assay.

Nanosheets of reduced graphene oxide were functionalized with silk peptide and used to immobilize anti-PSA to fabricate an immunosensor for PSA.

     

Direct electrochemistry of glucose oxidase and sensing glucose using a screen-printed carbon electrode modified with graphite nanosheets and zinc oxide nanoparticles

We have studied the direct electrochemistry of glucose oxidase (GOx) immobilized on electrochemically fabricated graphite nanosheets (GNs) and zinc oxide nanoparticles (ZnO) that were deposited on a screen printed carbon electrode (SPCE). The GNs/ZnO composite was characterized by using scanning electron microscopy and elemental analysis. The GOx immobilized on the modified electrode shows a well-defined redox couple at a formal potential of −0.4 V. The enhanced direct electrochemistry of GOx (compared to electrodes without ZnO or without GNs) indicates a fast electron transfer at this kind of electrode, with a heterogeneous electron transfer rate constant (K_s) of 3.75 s⁻¹. The fast electron transfer is attributed to the high conductivity and large edge plane defects of GNs and good conductivity of ZnO-NPs. The modified electrode displays a linear response to glucose in concentrations from 0.3 to 4.5 mM, and the sensitivity is 30.07 μA mM⁻¹ cm⁻². The sensor exhibits a high selectivity, good repeatability and reproducibility, and long term stability.

Graphical representation for the fabrication of GNs/ZnO composite modified SPCE and the immobilization of GOx

     

Highly sensitive and wide-range nonenzymatic disposable glucose sensor based on a screen printed carbon electrode modified with reduced graphene oxide and Pd-CuO nanoparticles

A nanocomposite consisting of reduced graphene oxide decorated with palladium-copper oxide nanoparticles (Pd-CuO/rGO) was synthesized by single-step chemical reduction. The morphology and crystal structure of the nanocomposite were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction analysis. A 3-electrode system was fabricated by screen printing technology and the Pd-CuO/rGO nanocomposite was dropcast on the carbon working electrode. The catalytic activity towards glucose in 0.2 M NaOH solutions was analyzed by linear sweep voltammetry and amperometry. The steady state current obtained at a constant potential of +0.6 V (vs. Ag/AgCl) showed the modified electrode to possess a wide analytical range (6 μM to 22 mM), a rather low limit of detection (30 nM), excellent sensitivity (3355 μA∙mM⁻¹∙cm⁻²) and good selectivity over commonly interfering species and other sugars including fructose, sucrose and lactose. The sensor was successfully employed to the determination of glucose in blood serum.

A highly sensitive nonenzymatic electrochemical sensor was fabricated using a Pd-CuO composite with reduced graphene oxide. The sensor has a wide detection range and was used to sense glucose in blood serum

     

Ultrasensitive non-enzymatic glucose sensing at near-neutral pH values via anodic stripping voltammetry using a glassy carbon electrode modified with Pt3Pd nanoparticles and reduced graphene oxide

We describe an anodic stripping voltammetric (ASV) method for glucose sensing that widely expands the typical amperometric i-t response of glucose sensors. The electrode is based on a working electrode consisting of a glassy carbon electrode modified with Pt-Pd nanoparticles (NPs; in an atomic ratio of 3:1) on a reduced graphene oxide (rGO) support. The material was prepared via the spontaneous redox reaction between rGO, PdCl₄ ²⁻ and PtCl₄ ²⁻ without any additional reductant or surfactant. Unlike known Pt-based sensors, the use of Pt₃Pd NPs results in an ultrasensitive ASV approach for sensing glucose even at near-neutral pH values. If operated at a working voltage as low as 0.06 V (vs. SCE), the modified electrode can detect glucose in the 2 nM to 300 μM concentration range. The lowest detectable concentration is 2 nM which is much lower than the LODs obtained with other amperometric i-t type sensing approaches, most of which have LODs at a μM level. The sensor is not interfered by the presence of 0.1 M of NaCl.

We describe an anodic stripping voltammetric method for glucose sensing that widely expands the typical amperometric i-t response of glucose sensors (2 nM to 300 μM). The electrode is based on a glassy carbon electrode modified with Pt-Pd nanoparticles on a reduced graphene oxide (rGO) support.

     

Electrochemical determination of dopamine using octahedral SnO2 nanocrystals bound to reduced graphene oxide nanosheets

Novel nanocomposites were prepared from graphene oxide (GO) and octahedral tin dioxide (SnO₂) through a facile process that included synthesis of octahedral SnO₂ and the reduction of GO with ascorbic acid. The morphology and structure of the nanocomposites were characterized by UV–vis spectroscopy, transmission electron microscopy, and Raman spectroscopy. The nanocomposites were placed on a glassy carbon electrode where they displayed excellent performance in terms of differential pulse voltammetric determination of dopamine (DA). This is attributed to (a) the synergetic interactions between reduced graphene oxide (r-GO) and octahedral SnO₂, and (b) the presence of a large number of active sites on the nanocomposites surface. The sensor responds to DA in the concentration range of 0.08 to 30 μM, with a 6 nM detection limit if operated at 0.24 V (vs. Ag/AgCl). The modified electrode also widely suppresses the background current resulting from excess ascorbic acid and uric acids. The method was applied to the determination of DA in spiked human urine and gave satisfactory results, with recoveries in the range from 96.4 to 98.2 %.

Green and facile synthesis of reduced graphene oxide-octahedral SnO₂ (r-GO-SnO₂) nanocomposites for the sensitive and selective electrochemical detection of dopamine.

     

A nanocomposite consisting of plasma-polymerized propargylamine and graphene for use in DNA sensing

We report on a novel graphene-based nanoarchitecture modified with plasma-polymerized propargylamine (G-PpPG) and its application in electrochemical sensors for DNA. Films of G-PpPG were characterized by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The presence of graphene enhances the electrochemical activity of the films, and the high density of amino groups (deposited at a low plasma input power) on their surface assists in the immobilization of probe DNA on the water-swollen polymeric network. By contrast, the degree of hybridization of the total complementary target DNA to the probe DNA remains unchanged when G-PpPG nanofilms prepared at higher input power. No substantial non-specific adsorption of totally mismatched target DNA on the polymer films is observed because of the complete coverage of the probe DNA. The detection limit for total complementary target DNA is approximately 1.84 nmol · L⁻¹. The dynamic range extends from 0.1 to 1,000 nmol · L⁻¹. The new nanocomposite may also be used to immobilize other probe DNA sequences, and this makes the approach potentially applicable to the detection of other oligomers.

Preparing the DNA sensor made from the graphene-based nanoarchitecture modified by using PpPG (G-PpPG) includes the following processes: (a) Modifying the Au electrode with the graphene nanosheet, (b) depositing the PpPG film onto the Au electrode coated with graphene, (c) immobilizing the probe DNA onto the G-PpPG film, and (d) hybridizing the MM0 target with the G-PpPG film immobilized with P1

     

Simultaneous sensing of L-tyrosine and epinephrine using a glassy carbon electrode modified with nafion and CeO2 nanoparticles

An electrochemical sensor was developed and tested for detection of L-tyrosine in the presence of epinephrine by surface modification of a glassy carbon electrode (GCE) with Nafion and cerium dioxide nanoparticles. Fabrication parameters of a surfactant-assisted precipitation method were optimized to produce 2–3 nm CeO₂ nanoparticles with very high surface-to-volume ratio. The resulting nanocrystals were characterized structurally and morphologically by X-ray diffractometery (XRD), scanning and high resolution transmission electron microscopy (SEM and HR-TEM). The nanopowder is sonochemically dispersed in a Nafion solution which is then used to modify the surface of a GCE electrode. The electrochemical activity of L-tyrosine and epinephrine was investigated using both a Nafion-CeO₂ coated and a bare GCE. The modified electrode exhibits a significant electrochemical oxidation effect of L-tyrosine in a 0.2 M Britton-Robinson (B-R) buffer solution of pH 2. The electro-oxidation peak current increases linearly with the L-tyrosine concentration in the molar concentration range of 2 to 160 μM. By employing differential pulse voltammetry (DPV) for simultaneous measurements, we detected two reproducible peaks for L-tyrosine and epinephrine in the same solution with a peak separation of about 443 mV. The detection limit of the sensor (signal to noise ratio of 3) for L-tyrosine is ~90 nM and the sensitivity is 0.20 μA μM⁻¹, while for epinephrine these values are ~60 nM and 0.19 μA μM⁻¹. The sensor exhibited excellent selectivity, sensitivity, reproducibility and stability as well as a very good recovery time in real human blood serum samples.

Simultaneous electrochemical determination of L-tyrosine and epinephrine in blood plasma with Nafion-CeO₂/GCE modified electrode showing a 443 mV peak-to-peak potential difference between species oxidation peak currents.

     

Methyltransferase activity assay based on the use of exonuclease III,the hemin/G-quadruplex system and reduced graphene oxide on a gold electrode,and a study on enzyme inhibition

We describe an electrochemical bioassay for the detection of the activity of methyltransferase (MTase), and for screening this enzyme’s inhibitors. The assay is based on the conjugation of a hemin to a G-quadruplex that enables enzymatic signal amplification with the aid of exonuclease III (ExoIII). In the first step, double-stranded DNA containing the quadruplex-forming oligomer is assembled on the surface of a gold electrode and then methylated by DNA adenine methyltransferase (DAM). After cleaved by endonuclease DpnI, the methylated DNA is digested by ExoIII and the quadruplex-forming oligomers are liberated. This leads to the formation of a hemin/G-quadruplex (in presence of hemin and of potassium ions). The hemin/G-quadruplex catalyzes the oxidization of hydroquinone by H₂O₂ and the benzoquinone was formed to generate electrochemical signal. Finally, the gold electrode modified with reduced graphene oxide was used as working electrode for performing differential pulse voltammetry. The method has a detection limit of 0.31 unit · mL⁻¹. A study on the inhibition of MTase showed it was inhibited by epicatechin with an IC50 value of 157 μM.

We describe an electrochemical bioassay for the detection of the activity of methyltransferase and for screening for its inhibitors. Due to the conjugation of a hemin to a G-quadruplex, strong enzymatic signal amplification is enabled with the aid of exonuclease III.

     

Electrochemical sandwich immunoassay for the peptide hormone prolactin using an electrode modified with graphene,single walled carbon nanotubes and antibody-coated gold nanoparticles

We describe a new kind of electrochemical immunoassay for the peptide hormone prolactin. A glassy carbon electrode (GCE) was modified with a hybrid material consisting of graphene, single walled carbon nanotubes and gold nanoparticles (AuNPs) in a chitosan (CS) matrix. The graphene and the single wall carbon nanotubes were first placed on the GCE, and the AuNPs were then electrodeposited on the surface by cyclic voltammetry. This structure results in a comparably large surface for immobilization of the capturing antibody (Ab₁). The modified electrode was used in a standard sandwich-type of immunoassay. The secondary antibody (Ab₂) consisted of AuNPs with immobilized Ab₂ and modified with biotinylated DNA as signal tags. Finally, alkaline phosphatase was bound to the biotinylated DNA-AuNPs-Ab₂ conjugate via streptavidin chemistry. The enzyme catalyzes the hydrolysis of the α-naphthyl phosphate to form α-naphthol which is highly electroactive at an operating voltage as low as 180 mV (vs. Ag/AgCl). The resulting immunoassay exhibits high sensitivity, wide linear range (50 to 3200 pg∙mL‾¹), low detection limit (47 pg∙mL‾¹), acceptable selectivity and reproducibility. The assay provides a pragmatic platform for signal amplification and has a great potential for the sensitive determination of antigens other than prolactine.

The immunoassay for prolactin is based on a glassy carbon electrode modified with SWCNTs, graphene and antibody-coated gold nanoparticles, and a secondary antibody conjugated to other gold nanoparticles via a biotinylated DNA linker

     

Nonenzymatic sensing of glucose at neutral pH values and low working potential using a glassy carbon electrode modified with platinum-iron alloy nanoparticles on a carbon support

Alloy nanoparticles of the type Pt_xFe (where x is 1, 2 or 3) were synthesized by coreduction with sodium borohydride in the presence of carbon acting as a chemical support. The resulting nanocomposites were characterized by scanning electron microscopy and X-ray diffraction. The nanocomposite was placed on a glassy carbon electrode, and electrochemical measurements indicated an excellent catalytic activity for the oxidation of glucose even a near-neutral pH values and at a working voltage as low as 50 mV (vs. SCE). Under optimized conditions, the sensor responds to glucose in the 10.0 μM to 18.9 mM concentration range and with a 3.0 μM detection limit (at an S/N ratio of 3). Interferences by ascorbic acid, uric acid, fructose, acetamidophenol and chloride ions are negligible.

Nonenzymatic sensing of glucose is demonstrated at neutral pH values and low working potential using a glassy carbon electrode modified with platinum-iron alloy nanoparticles on a carbon support.

     

Sensitive immunoassay for the β-agonist ractopamine based on glassy carbon electrode modified with gold nanoparticles and multi-walled carbon nanotubes in a film of poly-arginine

We are presenting an electrochemical immunosensor for the determination of the β-agonist and food additive ractopamine. A glassy carbon electrode (GCE) was modified with gold nanoparticles and a film of a composite made from poly(arginine) and multi-walled carbon nanotubes. Antibody against ractopamine was immobilized on the surface of the modified GCE which then was blocked with bovine serum albumin. The assembly of the immunosensor was followed by electrochemical impedance spectroscopy. Results demonstrated that the semicircle diameter increases, indicating that the film formed on the surface hinders electron transfer due to formation of the antibody-antigen complex on the modified electrode. Under optimal conditions, the peak current obtained by differential pulse voltammetry decreases linearly with increasing ractopamine concentrations in the 0.1 nmol•L⁻¹ to 1 μmol•L⁻¹ concentration range. The lower detection limit is 0.1 nmol•L⁻¹. The sensor displays good stability and reproducibility. The method was applied to the analysis of spiked swine feed samples and gave satisfactory results.

Immunoassay for ractopamine based on glassy carbon electrode modified with gold nanoparticles and a film of a composite made from poly (arginine) and multi-walled carbon nanotubes was proposed. Under optimal conditions, the peak currents obtained by differential pulse voltammetry decreases linearly with increasing ractopamine concentrations in the 0.1 nmol•L⁻¹ to 1 μmol•L⁻¹ concentration range. The detection limit is 0.1 nmol•L⁻¹.

     

Enhanced amperometric response of a glucose oxidase and horseradish peroxidase based bienzyme glucose biosensor modified with a film of polymerized toluidine blue containing reduced graphene oxide

Reduced graphene oxide (RGO) was used to construct a bienzyme biosensor containing horseradish peroxidase (HRP) and glucose oxidase (GOx). A poly(toluidine blue) (pTB) film containing RGO acted as both enzyme immobilization matrix and electron transfer mediator. The bienzyme biosensor was characterized by electrochemical techniques and displays a highly sensitive amperometric response to glucose and hydrogen peroxide (H₂O₂) at a potential as low as −0.1 V (vs. SCE). It is shown that use of RGO causes a strong enhancement on the amperometric responses. H₂O₂ formed by the action of GOx in the presence of oxygen can be further reduced by HRP in the pTB film contacting the RGO modified electrode. In the absence of oxygen, glucose oxidation proceeds by another mechanism in which electron transfer occurs from GOx to the electrode and with pTB acting as the mediator. Amperometric responses to glucose and H₂O₂ follow Michaelis-Menten kinetics. The experimental conditions were optimized, and under these conditions glucose can be determined in the 80 μM to 3.0 mM range with a detection limit of 50 μM. H₂O₂, in turn, can be quantified in up to 30.0 μM concentration with a detection limit of 0.2 μM. The bienzyme biosensor is reproducible, repeatable and stable. Finally, it has been successfully applied to the determination of glucose in plasma samples.

Schematic representation of glocuse detection at GCE/RGO/pTB-HRP-GOx.

     

Electrocatalytic sensing of hydrogen peroxide using a screen printed carbon electrode modified with nitrogen-doped graphene nanoribbons

We have synthesized nitrogen-doped graphene nanoribbons (N-GrNRs) by unzipping multi-walled carbon nanotubes (CNTs) under strongly oxidizing conditions and subsequent doping with nitrogen by a low-temperature hydrothermal method. The N-GNRs were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy, and assembled on a disposable screen-printed carbon electrode to give a sensor for H₂O₂ that was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, chronocoulometry and chronoamperometry. The nano-modified electrode displays enhanced electron transfer ability, and has a large active surface and a large number of catalytically active sites that originate from the presence of nitrogen atoms. This results in a catalytic activity towards H₂O₂ reduction at near-neutral pH values that is distinctly improved compared to electrodes modified with CNTs or unzipped (non-doped) CNTs only. At a working potential of −0.4 V (vs. Ag/AgCl), the amperometric responses to H₂O₂ cover the 5 to 2785 μM concentration range, with a limit of detection as low as 1.72 μM. This enzyme-free electrochemical sensor exhibits outstanding selectivity and long-term stability for H₂O₂ detection.

Nitrogen-doped graphene nanoribbons (N-GrNRs) were expediently synthesized for highly sensitive and selective detection of H₂O₂.

     

Preparation of MIP grafts for quercetin by tandem aryl diazonium surface chemistry and photopolymerization

The food antioxidant quercetin was used as a template in an ultrathin molecularly imprinted polymer (MIP) film prepared by photopolymerization. Indium tin oxide (ITO) plates were electrografted with aryl layers via a diazonium salt precursor bearing two terminal hydroxyethyl groups. The latter act as hydrogen donors for the photosensitizer isopropylthioxanthone and enabled the preparation of MIP grafts through radical photopolymerization of methacrylic acid (the functional monomer) and ethylene glycol dimethacrylate (the crosslinker) in the presence of quercetin (the template) on the ITO. The template was extracted, and the remaining ITO electrode used for the amperometric determination of quercetin at a working potential of 0.26 V (vs. SCE). The analytical range is from 5.10⁻⁸ to 10⁻⁴ mol L⁻¹, and the detection limit is 5.10⁻⁸ mol L⁻¹.

This work describes the grafting of a molecularly imprinted polymer (MIP) film by combining diazonium surface chemistry and surface-initiated photopolymerization. The MIP grafts specifically and selectively recognize quercetin in pure solution in THF and in real green tea infusion.

     

Determination of trypsin activity using a gold electrode modified with a nanocover composed of graphene oxide and thionine

We report on an electrochemical method for the determination of the activity of trypsin. A multi-functional substrate peptide (HHHAKSSATGGC-HS) is designed and immobilized on a gold electrode. The three His residues in the N-terminal are able to recruit thionine-loaded graphene oxide (GO/thionine), a nanocover adopted for signal amplification. Once the peptide is cleaved under enzymatic catalysis by trypsin (cleavage site: Lys residue), the His residues leave the electrode, and the GO/thionine cannot cover the peptide-modified electrode anymore. Thus, the changes of the electrochemical signal of thionine, typically acquired at a voltage of -0.35 V, can be used to determine the activity of trypsin. A detection range of 1 × 10⁻⁴ to 1 U, with a detection limit of 3.3 × 10⁻⁵ U, can be achieved, which is better than some currently available methods. In addition, the method is highly specific, facile, and has the potential for the detection of trypsin-like proteases.

Graphene oxide was adopted as a nanocover for the development of a sensitive electrochemical method to detect the activity of trypsin.

     

Magnetic molecularly imprinted polymer nanoparticles for the solid-phase extraction of paracetamol from plasma samples,followed its determination by HPLC

We are presenting magnetic molecularly imprinted polymer nanoparticles (m-MIPs) for solid-phase extraction and sample clean-up of paracetamol. The m-MIPs were prepared from magnetite (Fe₃O₄) as the magnetic component, paracetamol as the template, methacrylic acid as a functional monomer, and 2-(methacrylamido) ethyl methacrylate as a cross-linker. The m-MIPs were then characterized by transmission electron microscopy, FT-IR spectroscopy, X-ray diffraction and vibrating sample magnetometry. The m-MIPs were applied to the extraction of paracetamol from human blood plasma samples. Following its elution from the column loaded with the m-MIPs with an acetonitrile-buffer (9:1) mixture, it was submitted to HPLC analysis. Paracetamol can be quantified by this method in the 1 μg L⁻¹ to 300 μg L⁻¹ concentration range. The limit of detection and limit of quantification in plasma samples are 0.17 and 0.4 μg L⁻¹. The preconcentration factor of the m-MIPs is 40. The HPLC method shows good precision (4.5 % at 50 μg L⁻¹ levels) and recoveries (between 83 and 91 %) from spiked plasma samples.

We are presenting magnetic molecularly imprinted polymer nanoparticles (m-MIPs) for solid-phase extraction and sample clean-up of paracetamol. The m-MIPs were applied to the extraction of paracetamol from human blood plasma samples

     

A fluorescent turn-on detection scheme for α-fetoprotein using quantum dots placed in a boronate-modified molecularly imprinted polymer with high affinity for glycoproteins

This article describes a fluorescent molecularly imprinted polymer (MIP) capable of selective fluorescent turn-on recognition of the tumor biomarker α-fetoprotein. The technique is making use of amino-modified Mn-doped ZnS quantum dots (QDs) as solid supports, 4-vinylphenylboronic acid and methyl methacrylate as the functional monomers, γ-methacryloxypropyl trimethoxysilane as the grafting agent, and α-fetoprotein as a template. A graft imprint is created on the surface of the QDs. The functional monomers are shown to play an important role in the formation of the binding sites and in preventing nonspecific protein binding. The resulting MIP-QDs display a good linear response to α-fetoprotein in the 50 ng · L⁻¹ to 10 μg · L⁻¹ concentration range, and the limit of detection is 48 ng · L⁻¹. In our perception, the method has a wide scope in that it may be adapted to various other glycoproteins.

Schematic illustration of the synthesis of the MIP-QDs composites