Nanomaterial Modification

The surface of materials affects their properties, and this is particularly obvious for nanomaterials, which have a larger percentage of atom surfaces than bulk materials. Surface studies of nanomaterials have attracted great interest from researchers in the fields of chemistry, physics, and materials science, owing to their great academic value and potential applications^[1]. All along, researchers have focused on exploiting existing chemical reactions or finding new synthetic methods to modify various nanomaterials. Most of these methods use self-assembled monolayers (SAMs) on metal and metal oxide surfaces, and others often based on complexation and covalent modifications. However, none of above methods is universal because the preliminary surface modification step allows the attachment of small molecules, polymers, and complex ligands^[2]. The concept of click chemistry, especially copper (I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction, represents a high possibility for nanomaterial modification. The schematic diagram of nanomaterial modification by CuAAC click reaction is shown in scheme 1.

Scheme 1. Schematic diagram of nanomaterial modification by CuAAC click reaction.

Applications

• Surface modification of inorganic nanomaterials

Click chemistry has been widely used in the surface modification of inorganic nanomaterials. To date, gold nanoparticles (NPs) are the most intensively studied of all inorganic nanomaterials whose surfaces have been modified by click chemistry. In 2017, Liu et al^[3]. developed folate receptor (FR)-targeted surface-enhanced Raman scattering (SERS) nanoprobes for cancer cell imaging based on a copper-free click reaction between azide and BCN (Scheme 2). They synthesized hollow AuNPs that enhance Raman signals, and modified them with azide-labeled 5,5'-dithiobis (2-nitrobenzoic acid). Then, the azide groups on nanoparticle surfaces were conjugated to folate bicyclo[6.1.0]nonyne (BCN) derivatives, a kind of strain-promoted alkynes. Apart from AuNPs, click chemistry can also be applied to the surface modification of silicon-based NPs, carbon nanotubes (CNTs), magnetic NPs (MNPs), and AgNPs.

Scheme 2. Modification of SERS nanoprobes with folate by copper-free click
reaction between azide and BCN for cancer cell imaging.

• Surface modification of organic nanomaterials

Click chemistry also shows important applications in the surface modification of organic nanomaterials. Liu and co-workers synthesized the surface "clickable" shell cross-linked (SCL) micelles by using an alkynyl-terminated functional triblock copolymer. This new kind of four layer NPs could be further modified or conjugated with other azido terminated polymer chains, functional groups, or biomolecules via click chemistry (Scheme 3), and can be used as nano-sized drug delivery vehicles^[4].

Alkynyl-POEGMA-b-PDMA-b-PDEA:
poly(oligo(ethylene glycol) monomethyl
ether methacrylate)-b-poly(2-(dimethylamino)
ethyl methacrylate)-b-poly(2-(diethylamino)
ethyl methacrylate).

Scheme 3. Schematic diagram of the preparation of surface alkynyl functionalized SCL micelles from
alkynyl-POEGMA-b-PDMA-b-PDEA triblock copolymer and the subsequent fabrication of
PNIPAM-functionalized SCL micelles (SCL-PNIPAM) via click chemistry.

What Can We Do?

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References

1. Hea, H.; Gao, C. Click chemistry on nano-surfaces. Current Organic Chemistry. 2011, 15: 3667-3691.
2. Lia, N.; Binder, W.H. Click-chemistry for nanoparticle-modification. Journal of Materials Chemistry. 2011, 21: 16717.
3. Liu, R.; et al. Click-functionalized SERS nanoprobes with improved labeling efficiency and capability for cancer cell imaging. ACS Applied Materials & Interfaces. 2017, 9(44): 38222-38229.
4. Jiang, X.; et al. Covalently stabilized temperature and pH responsive four-layer nanoparticles fabricated from surface 'clickable' shell cross-linked micelles. Soft Matter. 2009, 5: 1530-1538.