Biofunctionalization on Alkylated Silicon Substrate Surfaces via “Click” Chemistry

Abstract

Biofunctionalization of silicon substrates is important to the development of silicon-based biosensors and devices. Compared to conventional organosiloxane films on silicon oxide intermediate layers, organic monolayers directly bound to the non-oxidized silicon substrates via Si-C bonds enhance the sensitivity of detection and the stability against hydrolytic cleavage. Such monolayers presenting a high density of terminal alkynyl groups for bioconjugation via coppercatalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC, a "click" reaction) were reported. However, yields of the CuAAC reactions on these monolayer platforms were low. Also, the nonspecific adsorption of proteins on the resultant surfaces remained a major obstacle for many potential biological applications. Herein, we report a new type of "clickable" monolayers grown by selective, photo-activated surface hydrosilylation of α,ω-alkenynes, where the alkynyl terminal is protected with a trimethylgermanyl (TMG) group, on hydrogen-terminated silicon substrates. The TMG groups on the film are readily removed in aqueous solutions in the presence of Cu(I). Significantly, the degermanylation and the subsequent CuAAC reaction with various azides could be combined into a single step in good yields. Thus, oligo(ethylene glycol) (OEG) with an azidotag was attached to the TMG-alkyne surfaces, leading to OEG-terminated surfaces that reduced the non-specific adsorption of protein (fibrinogen) by >98%. The CuAAC reaction could be performed in microarray format to generate arrays of mannose and biotin with varied densities on the protein-resistant OEG background. We also demonstrated that the monolayer platform could be functionalized with mannose for highly specific capturing of living targets (Escherichia coli expressing fimbriae) onto the silicon substrates.

Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 terminated Si(111) surfaces (CH 3 CC-Si(111)) exhibit nearly complete termination of the Si(111) surface while maintaining a relatively low, air-stable, surface recombination velocity. [24][25][26][27][28][29][30] Previous characterization of CH 3 CC-Si(111) surfaces has also indicated that the CH 3 CC-groups are oriented normal to the surface by infrared spectroscopy, the surfaces exhibit a (1 × 1) surface unit cell by low-energy electron diffraction, and the surfaces exhibit broad atomic terraces by atomic-force microscopy. 24 However, a full understanding of the chemical and electronic structures of Si surfaces functionalized with alkynyl moieties is still lacking, owing to the paucity of surface-sensitive spectroscopic techniques suitable for the detailed characterization of molecular monolayers.…”

Click Chemistry‐Based Functionalization on Non‐Oxidized Silicon Substrates

“…Though Ag + was more efficient for the degermenylation, it did not promote the subsequent click reaction. [77] Using the above "clickable" platform, the OEG-azide (c in Scheme 8) was immobilized on the Si À C-bound organic monolayer. The protein resistance of the resultant OEGmodified film was evaluated by XPS measurement.…”

“…Consequently, this OEG-containing reactive monolayer represents a general starting point for making biofunctionalized silicon surfaces. [77] Due to the bioorthogonal properties of azide and alkyne, an added benefit of the CuAAC reaction is that it facilitates the simultaneous functionalization of multiple bioactive molecules, thereby making the study of a wide range of biological events possible. For instance, the reaction on the "clickable" silicon substrate could be performed in microarray format to attach azido-labeled bioactive components with varied densities on a bioinert OEG background to allow specific binding of targeted molecules.…”

“…We also demonstrated that the monolayer platforms could be functionalized with mannose to specifically capture living targets (Escherichia coli expressing mannose-binding fimbria) onto the silicon substrates, which may have potential applications in the fabrication of advanced pathogen-detection platforms. [77] …”

“…In contrast, some small and unhindered amines, such as ligand TMEDA (Scheme 7) used by Gooding et al, seems quite effective to increase the reaction rate and improve the coupling yield. [57,61] On our TMG-alkyne-terminated surface, we also tested several ligands with triazole moieties, [77,85,86] and identified the most efficient ligand TA-EG4 (Scheme 8). The OEG chain renders the Cu I complex water soluble, and the electron-donating NHCH 3 group maintains a high catalytic activity of the complex.…”