Liposome-Based Immunosensors. 1. Influence of Hapten Spacer Length on Liposome Binding Efficiency

Combining 3D graphene-like screen-printed carbon electrode with methylene blue-loaded liposomal nanoprobes for phospholipase A2 detection

Phospholipase A2 (PLA2) enzyme could be acted as a unique biomarker for forecasting and diagnosing certain diseases. Therefore, it is important to monitor PLA2 activity in biological and clinical samples. In this work, a simple electrochemical assay for PLA2 activity was developed based on a screen-printed carbon electrode (SPCE) with 3D graphene-like surface. When the PLA2-containing sample was mixed with the nanoprobes, i.e. the electroactive marker methylene blue (MB) encapsulated within nanometer-sized phospholipid liposomes, MB was released and adsorbed/enriched in site onto the surface of SPCE in a micro-cell. The encapsulation and enzymatic release of MB were evaluated using UV-Vis and fluorescence. The peak current due to oxidation of the adsorbed MB on the SPCE was measured by square-wave voltammetry (SWV). The current was directly linear to the PLA2 activity from 5 U/L to 200 U/L with a detection limit of 3 U/L. The same method can also be used for screening PLA2 inhibitors. Thus, the enrichment strategy developed in this work could be a promising signal amplification method for the sensitive and selective detection of PLA2 in biological or clinical samples.

Atomic force microscopy observation of highly arrayed phospholipid bilayer vesicle on a gold surface

Tapping mode atomic force microscopy (TM-AFM) imaging of a phospholipid bilayer vesicle (liposome) immobilized on a gold surface was performed to investigate morphologies of the electrode surfaces produced through application of three different sample preparation methods. We compared both methods from a morphological viewpoint using TM-AFM images. Liposomes, composed of zwitterionic and anionic phospholipids, were prepared by extrusion. Results indicate that the surface with immobilized L1-liposome, which was fabricated by the amino coupling method, seemed to form large amounts of aggregated or fused liposomes. In contrast, L2-liposome-containing 1-octadecanthiol that was directly attached on the gold surface using thiol-gold binding force was immobilized as a uniform surface topology without liposome aggregation. Finally, we attempted to arrange individual L3-liposome, prepared by mixing zwitterionic and anionic phospholipids, onto the gold layer by electron-beam (e-beam) lithography technique. A third method, L3-liposome formation on the sensor surface, is greatly anticipated for biosensor applications.

New antibody immobilization method via functional liposome layer for specific protein assays

A specific protein assay system based on functional liposome-modified gold electrodes has been demonstrated. To fabricate such assay system, a liposome layer was initially grown on top of a gold layer. The liposome layer contained two kinds of functional molecules: biotin molecules for the binding sites of streptavidin and N-(10,12-pentacosadiynoic)-acetylferrocene molecules for the facile redox probe in electrochemical detections. Then, streptavidin was attached on the functional liposme-modified layer using the interaction of streptavidin-sbiotin complex. On the streptavidin-attached surface, antibody molecules, anti-human serum albumin antibodies could be immobilized without any secondary antibodies. AFM imaging of the streptavidin-attached liposome surface revealed a uniform distribution of closely packed streptavidin molecules. In situ quartz-crystal microbalance and electrochemical measurements demonstrated that the wanted antibody-antigen reactions should occur with high specificity and selectivity. Our specific antibody assay system, based on a functional liposome modified electrode, can be developed further to yield sophisticated structures for numerous protein chips and immunoassay sensors.

Development of a new immunosensor for pesticide detection: a disposable system with liposome-enhancement and amperometric detection

The development of a disposable amperometric immunomigration sensor for the detection of triazine pesticides in real samples is presented. Thick film electrodes printed on PVC were used as strip-type transducers. Monoclonal antibodies against atrazine and terbutylazine served as biorecognition element. For the generation and amplification of the signal, hapten-tagged liposomes entrapping ascorbic acid as a marker molecule were chosen. A capillary gap and a wicking filter membrane strip served as migration zone. For signal detection on a graphite electrode, liposomes were lysed by Triton X-100 and the released ascorbic acid was quantified at a potential of + 300 mV vs printed Ag/AgCl. Signal response time was 1-3 min, and sensitivity of measurements in tap water was below 1 microgram l-1 of atrazine and terbutylazine. Atrazine determinations in soil extracts correlated well with standard procedures based on ELISA and HPLC.