Functionalization of atomic force microscope (AFM) tips with bioligands converts them into monomolecular biosensors which can detect complementary receptor molecules on the sample surface. Flexible PEG tethers are preferred because the bioligand can freely reorient and locally palpate the sample surface while the AFM tip is moved along. In a well-established coupling scheme [Hinterdorfer et al. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 3477-3481], a heterobifunctional PEG linker is used to tether thiol-containing bioligands to amino-functionalized AFM tips. Since antibodies contain no free thiol residues, prederivatization with N-succinimidyl 3-(acetylthio)propionate (SATP) is needed which causes a relatively high demand for antibody. The present study offers a convenient alternative with minimal protein consumption (e.g., 5 microg of protein in 50 microL of buffer) and no prederivatization, using a new heterobifunctional cross-linker that has two different amino-reactive functions. One end is an activated carboxyl (N-hydroxysuccinimide ester) which is much faster to react with the amino groups of the tips than the benzaldehyde function on its other end. The reactivity of the latter is sufficient, however, to covalently bind lysine residues of proteins via Schiff base formation. The method has been critically examined, using biotinylated IgG as bioligand on the tip and mica-bound avidin as complementary receptor. These experiments were well reproduced on amino-functionalized silicon nitride chips where the number of specifically bound IgG molecules (approximately 2000 per microm2) was estimated from the amount of specifically bound ExtrAvidin-peroxidase conjugate. For a bioscientific application, human rhinovirus particles were tethered to the tip, very-low-density lipoprotein receptor fragments were tethered to mica, and the specific interaction was studied by force microscopy.
We developed a new kind of suspension array for multiplexed immunoassays using silica colloidal crystal beads (SCCBs) as coding carriers. The monodisperse and size-controlled SCCBs were fabricated by a microfluidic device. Calcination was employed to improve the mechanical stability and lower the fluorescent background of the SCCBs. Immobilization of protein molecules on the surface of the SCCBs through chemical bonds was studied, and the modification condition was optimized to increase the detection sensitivity. Results indicated that the SCCBs as supports were more sensitive (0.92 ng/mL IgG) than the glass beads (27 ng/mL IgG) and the planar carriers (140 ng/mL IgG). A multiplex immunoassay showed the flexibility and feasibility of SCCBs array in clinical applications.
These data indicate that a stomatin-specific, raft-based process is involved in storage-associated vesiculation. A model of the vesiculation process in RBCs is proposed considering the raft-stabilizing properties of stomatin, the low storage temperature favoring raft aggregation, and the previously reported storage-associated changes in the cytoskeletal organization.
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