Immobilization of cholesterol oxidase in LbL films and detection of cholesterol using ac measurements

Paper-based optical sensors paired with smartphones for biomedical analysis

The traditional analytical methods used for biomedical analysis are expensive and not easy to handle and require sophisticated instruments, thus their application is limited in resource-limited settings. Due to their portability, low cost, and ability to be applied to different analytical techniques, paper-based analytical devices are becoming valuable tools for biomedical analysis. The integration of smartphones into analytical devices has provided the ability to build portable, cost-effective, straightforward analytical devices for biomedical analysis and mobile health. The key aim of this review is to emphasize the recent applications of PADs combined with a smartphone for the optical analysis of biomedical species. We started this review by highlighting the type of papers and their modifications with different materials to prepare the PADs. After that, this review presents various detection methods including colorimetry, fluorescence, and luminescence where the smartphone is used for read-out. In the end, we provided the recent applications of the analysis of different biomedical compounds such as cancer and cardiovascular biomarkers, metal ions, glucose, viruses, etc. We believe that the present review will attract a wide scientific community in the areas of analytical chemistry, sensors, and clinical testing.

Detecting cancer cells with a highly sensitive LbL-based biosensor

Early diagnosis of cancer is crucial for therapeutic methods to be more effective and to decrease the mortality rate due to this disease. Current diagnostic methods include imaging techniques that require expensive equipment and specialized personnel, making it difficult to apply them to many patients. To overcome these limitations, many biosensors have been developed to monitor cancer biomarkers. Here, we report on the electrochemical biosensor for selective detection of tumor cells using a simple and low-cost methodology. Layer-by-layer (LbL) self-assembly was used to modify indium tin oxide (ITO) electrodes with alternating layers of polyallylamine hydrochloride (PAH) and folic acid (FA), which binds to overexpressed folate receptors alpha (FRα) in tumor cells. The LbL-based biosensor showed high sensitivity in detecting cervical cancer cells (HeLa cells) using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). A linear dependence with the logarithm cell concentration was observed and excellent detection limits were found, 4 cells mL^-1 and 19 cells mL^-1 for EIS and CV measurements, respectively. The developed biosensor also presented great reproducibility (RSD = 1.7%) and repeatability (RSD = 1.8%). The selectivity was confirmed after the biosensor interaction with healthy cells (HMEC cells), which did not produce significant changes in the electrochemical signals. Furthermore, it was demonstrated that selective detection of tumor cells occurs via an interaction with FA. The LbL-based biosensor provides a simple, accurate, and cost-effective platform to be applied in the early diagnosis of cancer.

Protein-based polyelectrolyte multilayers

The immobilization of proteins to impart specific functions to surfaces is topical for chemical engineering, healthcare and diagnosis. Layer-by-Layer (LbL) self-assembly is one of the most used method to immobilize macromolecules on surfaces. It consists in the alternate adsorption of oppositely charged species, resulting in the formation of a multilayer. This method in principle allows any charged object to be immobilized on any surface, from aqueous solutions. However, when it comes to proteins, the promises of versatility, simplicity and universality that the LbL approach holds are unmet due to the heterogeneity of protein properties. In this review, the literature is analyzed to make a generic approach emerge, with a view to facilitate the LbL assembly of proteins with polyelectrolytes (PEs). In particular, this review aims at guiding the choice of the PE and the building conditions that lead to the successful growth of protein-based multilayered self-assemblies.

Electrochemical quartz crystal microbalance with dissipation investigation of fibronectin adsorption dynamics driven by electrical stimulation onto a conducting and partially biodegradable copolymer

Functional surface coatings are a key option for biomedical applications, from polymeric supports for tissue engineering to smart matrices for controlled drug delivery. Therefore, the synthesis of new materials for biological applications and developments is promising. Hence, biocompatible and stimuli-responsive polymers are interesting materials, especially when they present conductive properties. PEDOT-co-PDLLA graft copolymer exhibits physicochemical and mechanical characteristics required for biomedical purposes, associated with electroactive, biocompatible, and partially biodegradable properties. Herein, the study of fibronectin (FN) adsorption onto PEDOT-co-PDLLA carried out by an electrochemical quartz crystal microbalance with dissipation is reported. The amount of FN adsorbed onto PEDOT-co-PDLLA was higher than that adsorbed onto the Au surface, with a significant increase when electrical stimulation was applied (either at +0.5 or -0.125 V). Additionally, FN binds to the copolymer interface in an unfolded conformation, which can promote better NIH-3T3 fibroblast cell adhesion and later cell development.

Smartphone-based enzymatic biosensor for oral fluid L-lactate detection in one minute using confined multilayer paper reflectometry

The development of smartphone-based biosensors for point-of-care testing (POCT) applications allows realizing "all in one" instruments, with large potential distribution among the general population. With this respect, paper color-based detection performed by reflectance measurement is the most popular, simple, inexpensive and straightforward method. Despite the large number of scientific publications related to these biosensors, they still suffer from a poor detectability and reproducibility related to inhomogeneity of color development, which leads to low assay reproducibility. To overcome these problems, we propose a smartphone paper-based biosensor, in which all the reagents necessary to complete the analysis are co-entrapped on paper in a "wafer"-like bilayer film of polyelectrolytes (Poly (allyl amine hydrochloride/poly(sodium 4-styrene sulfonate)). Using a 3D printing low-cost technology we fabricated the smartphone-based device that consists in a cover accessory attached to the smartphone and incorporating a light diffuser over the flash to improve the image quality, a mini dark box and a disposable analytical cartridge containing all the reagents necessary for the complete analysis. The biosensor was developed exploiting coupled enzyme reactions for quantifying L-lactate in oral fluid, which is considered a biomarker of poor tissue perfusion, a key element in the management of severe sepsis, septic shock and in sports performance evaluation. The developed method is sensitive, rapid, and it allows detecting L-lactate in oral fluid in the relevant physiological range, with a limit of detection of 0.1mmolL^-1. The extreme simplicity of assay execution (no reagents need to be added) and flexibility of fabrication of the device, together with the high assay versatility (any oxidase can be coupled with HRP-based color change reaction) make our approach suitable for the realization of smartphone-based biosensors able to non-invasively detect a large variety of analytes of clinical interest.

Immobilization of chlorophyll by using layer-by-layer technique for controlled release systems and photodynamic inactivation

The development of systems for the controlled release of drugs is important because they allow the control of drug absorption and tissue distribution and also can reduce local toxicity. This study aimed to assemble and characterize two types of release systems, consisting of layer-by-layer films obtained from poly(allylamine) hydrochloride with chlorophyll (PAH/CHL films) or chlorophyll incorporated into dipalmitoylphosphatidylcholine liposomes (PAH/Lip+CHL films). For these systems, the molecular aggregation, growth process, thermally stimulated desorption, wettability, and controlling release of CHL was studied by using UV-vis spectroscopy and wetting contact angle analysis. In addition, experiments of photodynamic inactivation using PAH/CHL or PAH/Lip+CHL films with a 633-nm laser light were performed and the susceptibility of Candida albicans (C. albicans) to this approach was examined. Fluorescence and atomic force microscopies were used to investigate the surface morphology after the application of the photoinactivation procedure. A redshift of the UV-vis spectrum associated to films when compared with the spectrum of the CHL solution indicated a molecular aggregation of CHL molecules in the films. The film growth process was determined by a nucleation and a growth of spheroids or rods for either PAH/Lip+CHL or PAH/CHL films, respectively. Thermally activated desorption experiments indicated that interactions between CHL and PAH (126kJ/mol) in PAH/CHL or between Lip+CHL and PAH (140kJ/mol) in PAH/Lip+CHL films may be governed by electrostatic interactions. The wettability of PAH/Lip+CHL films was larger than that for PAH/CHL films, which can be attributed to hydrophilic groups on the surface of the DPPC liposomes. Release experiments revealed that free CHL in PAH/CHL films was released more slowly than its partner incorporated into liposomes. After the photodynamic inactivation, results of survival fraction and fluorescence microscopy revealed that C. albicans presented similar susceptibility for the two kinds of films. AFM supported the fluorescence one suggesting that cell death of C. albicans may occur due to damages to its cell wall by C. albicans.

Arming embolic beads with anti-VEGF antibodies and controlling their release using LbL technology

Transarterial chemoembolization (TACE) is used to treat various types of hypervascular tumors such as hepatocellular carcinoma and renal cancer. However, embolization and blocking of blood vessels nourishing a tumor mass evokes an angiogenic response due to the secretion of vascular endothelial growth factor (VEGF), which results in the formation of new blood vessels and eventually limitation in therapeutic efficacy. The presented work investigates the feasibility of loading the clinically used embolic beads (DC Bead®) with Bevacizumab (BEV), an anti-VEGF antibody, and control its release kinetics via Layer-by-Layer (LbL) coating. This strategy has the aim to achieve high, localized and sustained concentrations of BEV at the tumor site and reduce drug exposure in the systemic circulation. High loading of BEV on lyophilized beads of about 76mg BEV/bead vial was achieved. LbL coating was carried out by depositing alternating layers of the biocompatible polymers alginate and poly-L-lysine. Coating was proven successful by monitoring the reversal of zeta potential after addition of each layer. Morphological changes of the bead surface before and after coating were illustrated using SEM imaging. Moreover, release profiles from different formulations were studied and results showed that optimizing the number of deposited layers effectively slows the release of BEV for three days. Activity of released BEV was studied in different 2D and 3D cell based assays. Released BEV fractions showed comparable activity to fresh BEV solution used as control after 3days. In conclusion, our results suggest the opportunity for loading anti-VEGF antibodies on commercially available embolic beads to increase the efficacy of TACE of hypervascular tumors.

Bending of layer-by-layer films driven by an external magnetic field

We report on optimized architectures containing layer-by-layer (LbL) films of natural rubber latex (NRL), carboxymethyl-chitosan (CMC) and magnetite (Fe₃O₄) nanoparticles (MNPs) deposited on flexible substrates, which could be easily bent by an external magnetic field. The mechanical response depended on the number of deposited layers and was explained semi-quantitatively with a fully atomistic model, where the LbL film was represented as superposing layers of hexagonal graphene-like atomic arrangements deposited on a stiffer substrate. The bending with no direct current or voltage being applied to a supramolecular structure containing biocompatible and antimicrobial materials represents a proof-of-principle experiment that is promising for tissue engineering applications in biomedicine.

Enzymatic activity of cholesterol oxidase immobilized onto polymer nanoparticles mediated by Congo red

Poly(ethylene glycol), PEG, decorated polystyrene (PS) nanoparticles were synthesized and characterized by means of dynamic light scattering (DLS), zeta (ζ) potential measurements, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The adsorption of Congo red (CR) onto PS/PEG particles was evidenced by the decrease of ζ potential values and increase in the particles mean diameter in comparison to bare particles. Cholesterol oxidase (ChOx), the main enzyme in the oxidation of cholesterol, adsorbed onto PS/PEG and PS/PEG/CR particles, as revealed by the increase in the particles mean size and spectrophotometry. The enzymatic activity of free and immobilized ChOx was determined as a function of time by means of a coupled reaction with horseradish peroxidase. The activity of free ChOx decreased with time, while the activity of immobilized ChOx increased with time; after 1h reaction the latter was half of the former. Freeze-drying the ChOx covered PS/PEG/CR particles allowed their storage for at least one month under room conditions without loss of enzymatic activity. Conjugation effects between CR and ChOx or cholesterol evidenced by circular dichroism and spectrophotometry rendered a conformational state of ChOx, such that the enzymatic action was favored. ChOx adsorbed onto PS/PEG presents no enzymatic activity, probably due to ChOx denaturation or unfavorable orientation. Freeze-dried and freshly prepared dispersions of ChOx immobilized onto PS/PEG/CR particles yielded linear response in the cholesterol concentration range of 100mgdL(-1) (lowest limit of normal blood concentration) to 300mgdL(-1) (high risk level).

Immunosensor based on immobilization of antigenic peptide NS5A-1 from HCV and silk fibroin in nanostructured films

The peptide NS5A-1 (PPLLESWKDPDYVPPWHG), derived from hepatitis C virus (HCV) NS5A protein, was immobilized into layer-by-layer (LbL) silk fibroin (SF) films. Deposition was monitored by UV-vis absorption measurements at each bilayer deposited. The interaction SF/peptide film induced secondary structure in NS5A-1 as indicated by fluorescence and circular dichroism (CD) measurements. Voltammetric sensor (SF/NS5A-1) properties were observed when the composite film was tested in the presence of anti-HCV. The peptide-silk fibroin interaction studied here showed new architectures for immunosensors based on antigenic peptides and SF as a suitable immobilization matrix.

Efficient immobilization of glucose oxidase by in situ photo-cross-linking for glucose biosensing

A glucose biosensor was fabricated based on electrostatic self-assembly in combination with in situ photo-cross-linking of glucose oxidase (GOx) and diazoresin-chitosan (DAR-CS) on Prussian blue deposited multi-walled carbon nanotubes (PB-MWNTs) backbone. It was demonstrated that GOx was initially ionically deposited and subsequently covalently photo-cross-linked onto the PB-MWNTs backbone using photosensitive DAR-CS as the assembly interlayer. The modified electrode exhibited good electrical conductivity and effective electron transfer mediation toward H(2)O(2) reduction due to the employment of PB-MWNTs as the fabrication backbone. The biosensor showed high sensitivity of 77.9 μA mM(-1) cm(-2) to glucose in the linear concentration range from 1.0×10(-5) to 1.1×10(-3) M with fast response time of 10s, detection limit of 3.1×10(-6) M, and good anti-interference ability. More importantly, the biosensor exhibited greatly improved biosensing stability in comparison with the non-photo-cross-linked biosensor attributed to the conversion of weak ionic bonds to strong covalent ones for enzyme immobilization by the proposed strategy. The results for glucose determination in real serum samples with the biosensor were found to be in good agreement with those obtained by the conventional clinical procedure.

Recent advances in electronic tongues

This minireview describes the main developments of electronic tongues (e-tongues) and taste sensors in recent years, with a summary of the principles of detection and materials used in the sensing units. E-tongues are sensor arrays capable of distinguishing very similar liquids employing the concept of global selectivity, where the difference in the electrical response of different materials serves as a fingerprint for the analysed sample. They have been widely used for the analysis of wines, fruit juices, coffee, milk and beverages, in addition to the detection of trace amounts of impurities or pollutants in waters. Among the various principles of detection, electrochemical measurements and impedance spectroscopy are the most prominent. With regard to the materials for the sensing units, in most cases use is made of ultrathin films produced in a layer-by-layer fashion to yield higher sensitivity with the advantage of control of the film molecular architecture. The concept of e-tongues has been extended to biosensing by using sensing units capable of molecular recognition, as in films with immobilized antigens or enzymes with specific recognition for clinical diagnosis. Because the identification of samples is basically a classification task, there has been a trend to use artificial intelligence and information visualization methods to enhance the performance of e-tongues.

Preparation of a polypyrrole-polyvinylsulphonate composite film biosensor for determination of cholesterol based on entrapment of cholesterol oxidase

In this paper, a novel amperometric cholesterol biosensor with immobilization of cholesterol oxidase on electrochemically polymerized polypyrrole-polyvinylsulphonate (PPy-PVS) films has been accomplished via the entrapment technique on the surface of a platinum electrode. Electropolymerization of pyrrole and polyvinylsulphonate on the Pt surface was carried out by cyclic voltammetry between -1.0 and +2.0 V (vs. Ag/AgCl) at a scan rate of 100 mV upon the Pt electrode with an electrochemical cell containing pyrrole and polyvinylsulphonate. The amperometric determination is based on the electrochemical detection of H(2)O(2) generated in the enzymatic reaction of cholesterol. Determination of cholesterol was carried out by the oxidation of enzymatically produced H(2)O(2) at 0.4 V vs. Ag/AgCl. The effects of pH and temperature were investigated and optimum parameters were found to be 7.25 and 35 °C, respectively. The storage stability and operational stability of the enzyme electrode were also studied. The results show that 32% of the response current was retained after 19 activity assays. The prepared cholesterol biosensor retained 43% of initial activity after 45 days when stored in 0.1 M phosphate buffer solution at 4 °C.