Three-dimensional arrayed amino aerogel biochips for molecular recognition of antigens

Abstract

The three-dimensional (3D) biochips prepared in this study are composed of a glass microscopy slide arrayed with amino aerogel dots. The amino aerogel was produced using the sol–gel process, with an ionic liquid as the template followed by a solvent extraction to remove the template and build a three-dimensional mesoporous structure. The FTIR spectrum verified that the major template was removed and the ²⁹Si solid-state NMR spectra recognized the cross-linkages in the SiO₂ network structure. SEM images measured the particles at around 100 nm. After grinding, the BET analysis confirmed that the nano-size amino aerogel powders had exhibited specific surface area of 188 m²/g, pore volume of 0.83 cm³/g, and average pore size of 16.2 nm. The as-prepared amino aerogel surface contained amino functional groups capable of performing a sandwich immunoassay. The primary antibody was immobilized on the internal surface of the arrayed amino aerogel to capture its affinity antigen. On the top of the captured antigen, the report antibody was read its labeling fluorescent dye. In comparison to the corresponding two-dimensional (2D) biochip, the 3D amino aerogel biochips were observed to amplify signal intensities more effectively due to their remarkable capturing capability.

Introduction

Protein biochips are of major importance not only in disease detection and diagnosis [1], [2], [3], [4], but also in drug discovery [5] and medical therapy [6]. As a diagnostic tool, Protein biochips have advantages over DNA chips mainly because many diseases do not express a genetic signature. However, proteins are much more complex than DNA’s, in both their molecular structure and their sensitivity to environment. Proteins are generally known to be easily denaturized, causing conformation changes and bioactivity losses. In addition, the greatest drawback, in using protein chips as biosensors, is that protein cannot be as easily duplicated (or amplified in detection signals) as can DNA’s, using current polymerase-chain-reaction (PCR) techniques. Such a limitation significantly prevents protein biochips from becoming a popular diagnostic platform. However, the three-dimensional-structure (3D) biochips provide a feasible alternative to respond that critical issue.

Several reports detail the use of various materials, such as polyacrylamide gel [7], agarose [8], [9], dextran gel [10], and nitrocellulose [11], [12], [13], [14], to build three-dimensional protein biochips. The common superior property of these materials is their large internal surface area, which enables them to adsorb much more protein analytes than traditional 2D planar slides and to achieve higher detection sensitivity. Their hydrophilic surface allows the construction of a protein-friendly polymer, to allow better penetration of biomolecules into the 3D porous structure. Moreover, polymer hydrophilization gives a better wetting surface, which also favors bioanalytical performance [15].

Silica aerogel is a material with high porosity, large surface area, low density, and low thermal conductivity. These unique characteristics give this advanced material many applications in thermal insulation, electrical batteries, nuclear waste storage, catalysis, and acoustic insulation and as adsorbents [16]. Due to its chemical and mechanical robustness, large internal capacity, and biocompatibility, silica aerogel also has promise as a biocompatible scaffold to immobilize or protect biological materials in virus detection [17], protein entrapment [18], protein incorporation [19], hybridization array [20], [21], and building potential matrices in the future design of biosensors. Moreover, its use for capturing comet dust has launched this material into outer space applications (www.nasa.gov).

In our previous study [22], we have found that a modified silica aerogel could function as a recognition substrate for nucleotide acids because of its particular morphology. Nevertheless, as far as we understand, no study has reported using silica aerogel as a material for 3D arrayed protein biochips. This report proposes a preparation technique, which synthesizes 3D mesoporous amino silica aerogel at atmosphere from two silicon precursors, tetraethoxysilane (TEOS) and (3-aminopropyl)trimethoxysilane (APTS), by the sol–gel process using a ionic liquid (IL) as the solvent and template agent. The recyclable IL and atmosphere condition were expected to provide a cost-effective substrate material for biochips. The unique properties of IL, including negligible vapor pressure and ionicity, allow the hydrolysis and condensation of the sol–gel polymerization undertaken to produce a stable gel network without excess shrinkage and retain the aerogel structure after the solvent extraction and freeze-drying process. The as-prepared 3D amino silica aerogel biochips were used for the recognition of human interleukin-6 (IL6) and compared with corresponding 2D biochips.