Quantification of κ-casein in milk by an optical immunosensor

Rapid detection of mozzarella and feta cheese adulteration with cow milk through a silicon photonic immunosensor

Mozzarella di Bufala Campana and Feta are two cheeses with Protected Designation of Origin the fraudulent adulteration of which with bovine milk must be routinely checked to ensure that consumers actually buy these high-end products and avoid health issues related to bovine milk allergy. Here, we employed, for the first time, a silicon-based photonic immunosensor for the detection of mozzarella and feta adulteration with bovine milk. The photonic immunosensor used relies on Mach-Zehnder interferometers monolithically integrated along with their respective light sources on a silicon chip. A rabbit polyclonal antiserum raised against bovine κ-casein was used for the development of a competitive immunoassay realized in three steps, including a reaction with the antiserum, a biotinylated anti-rabbit IgG antibody, and streptavidin. The implementation of this assay configuration significantly reduced the non-specific signal due to the cheese matrix, and allowed completion of the assay in ∼9 min. After optimization of all assay conditions, bovine cheese could be quantified in mozzarella or feta at concentrations as low as 0.5 and 0.25% (w/w), respectively; both quantification limits were below the maximum allowable content of bovine milk in mozzarella and feta (1% w/w) according to the EU regulations. Equally important, the assays were reproducible with intra- and inter-assay coefficients of variation <10%, and exhibited a wide linear dynamic range that extended up to 50 and 25% (w/w) for mozzarella and feta, respectively. Taking into account its performance, the proposed immunosensor may be transformed to a new tool against fraudulent activities in the dairy industry.

Generation of High Affinity Anti-Peptide Polyclonal Antibodies Recognizing Goat αs1-Casein

The chemical, technological and allergy properties of goat's milk are significantly affected by the level of αs1-casein. Detection and quantification of αs1-casein requires high-specificity methods to overcome high-sequence similarity between this protein and others in the casein family. Unavailability of antibodies with high affinity and specificity towards goat αs1-casein hinders the development of immuno-based analytical methods such as enzyme-linked immunosorbent assay (ELISA) and biosensors. Here, we report the generation of polyclonal antibodies (or immunoglobulins, IgGs) raised towards goat αs1-casein N- (Nter) and C-terminal (Cter) peptide sequences. The Nter and Cter peptides of goat αs1-casein were immunized in rabbits for the generation of antisera, which were purified using protein G affinity chromatography. The binding affinity of the antisera and purified IgGs were tested and compared using indirect ELISA, where peptide-BSA conjugates and goat αs1-casein were used as the coating antigens. The Nter antiserum displayed higher titer than Cter antiserum, at 1/64,000 and 1/32,000 dilutions, respectively. The purification step further yielded 0.5 mg/mL of purified IgGs from 3 mL of antisera. The purified Nter IgG showed a significantly (p < 0.05) higher binding affinity towards peptide-BSA and goat αs1-casein, with lower Kd value at 5.063 × 10-3 μM compared to 9.046 × 10-3 μM for the Cter IgG. A cross-reactivity test showed that there was no binding in neither Nter nor Cter IgGs towards protein extracts from the milk of cow, buffalo, horse and camel. High-quality antibodies generated will allow further development of immuno-based analytical methods and future in vitro studies to be conducted on goat αs1-casein.

Assessment of goat milk adulteration with a label-free monolithically integrated optoelectronic biosensor

The label-free detection of bovine milk in goat milk through a miniaturized optical biosensor is presented. The biosensor consists of ten planar silicon nitride waveguide Broad-Band Mach-Zehnder interferometers (BB-MZIs) monolithically integrated and self-aligned with their respective silicon LEDs on the same Si chip. The BB-MZIs were transformed to biosensing transducers by functionalizing their sensing arm with bovine k-casein. Measurements were performed by continuously recording the transmission spectra of each interferometer through an external spectrometer. The amount of bovine milk in goat milk was determined through a competitive immunoassay by passing over the sensor mixtures of anti-k-casein antibodies with the calibrators or the samples. The output spectra of each BB-MZI recorded during the reaction were subjected to Discrete Fourier Transform in order to convert the observed spectral shifts to phase shifts in the wavenumber domain. The method had a detection limit of 0.04 % (v/v) bovine milk in goat milk, dynamic range 0.1-1.0 % (v/v), recoveries 93-110 %, and intra- and inter-assay coefficients of variation less than 12 and 15 %, respectively. The proposed biosensor compared well in terms of analytical performance with a competitive ELISA developed using the same monoclonal antibodies. Nevertheless, the duration of the biosensor assay was 10 min whereas the ELISA required 2 h. Thus, the fast and sensitive determinations along with the small size of the sensor make it ideal for incorporation into portable devices for assessment of goat or ewe's milk adulteration with bovine milk at the point-of-need.

Electrochemical immunosensor for casein based on gold nanoparticles and poly(L-Arginine)/multi-walled carbon nanotubes composite film functionalized interface

In this paper, a novel electrochemical immunosensor for the determination of casein based on gold nanoparticles and poly(L-Arginine)/multi-walled carbon nanotubes (P-L-Arg/MWCNTs) composite film was proposed. The P-L-Arg/MWCNTs composite film was used to modify glassy carbon electrode (GCE) to fabricate P-L-Arg/MWCNTs/GCE through electropolymerization of L-Arginine on MWCNTs/GCE. Gold nanoparticles were adsorbed on the modified electrode to immobilize the casein antibody and to construct the immunosensor. The stepwise assembly process of the immunosensor was characterized by cyclic voltammetry and differential pulse voltammetry. Results demonstrated that the peak currents of [Fe(CN)(6)](3-/4-) redox pair decreased due to the formation of antibody-antigen complex on the modified electrode. The optimization of the adsorption time of gold nanoparticles, the pH of supporting electrolyte and the incubation time were investigated in details. Under optimal conditions, the peak currents obtained by DPV decreased linearly with the increasing casein concentrations in the range from 1 × 10(-7) to 1 × 10(-5) g mL(-1) with a linear coefficiency of 0.993. This electrochemical immunoassay has a low detection limit of 5 × 10(-8) g mL(-1) and was successfully applied to the determination of casein in cheese samples.

Use of surface plasmon resonance (SPR) to study the dissociation and polysaccharide binding of casein micelles and caseins

Tests were made to determine whether surface plasmon resonance (SPR) could be used as a technique to study the dissociation properties of bovine casein micelles or of sodium caseinate and the interactions between these protein particles and different polysaccharides. Surfaces of bound micelles or caseinate were made, and the changes in refractive index of these layers were used to define changes in the structures of the chemisorbed material. The technique appears to have some potential for studying details of the dissociation of casein micelles and of the binding of different polysaccharides to caseins.

Surface plasmon resonance immunosensor for human cardiac troponin T based on self-assembled monolayer

The cardiac troponin T (cTnT) is specific biomarker important for trials of acute myocardial infarctions (AMI). In this paper, a SPR sensor in real time to detect the biomarker was developed on a commercially available surface plasmon resonance AUTOLAB SPIRIT. The cTnT receptor molecule was covalently immobilized on a gold substrate via a self-assembled monolayer (SAM) of thiols by using cysteamine-coupling chemistry. This biosensor presented a linear response range for cTnT between 0.05 and 4.5 ng/mL (r=0.997, p<<0.01) with a good reproducibility (CV=4.4%). The effect of the cysteamine (CYS) concentrations on the SAM coated gold sensor was studied as a function of the amount of the immobilized cTnT monoclonal antibodies. Analysis using serum samples undiluted was carried out at room temperature showing a well agreement with the ECLIA methods and the sensor surface could be regenerated by using a solution of 1% (w/v) sodium dodecyl sulphate (SDS) without losing the sensor immunoreactivity. These studies open new perspectives of using SAM to develop regenerable immunosensor with a good reproducibility allowing its use in the clinical applications.