(Invited) Near-Infrared Nanosensors Driven By Molecular Recognition at sp3 Defect Sites of Locally Functionalized Single-Walled Carbon Nanotubes
Authors: Shiraki Tomohiro 
Published: December 2024

Single-walled carbon nanotubes (SWCNTs) emit photoluminescence (PL) in the near-infrared (NIR) region. NIR PL is applicable to advanced biosensors and bioimaging because of its high transparency, low light scattering, and suppressed autofluorescence in biological tissues. Local chemical functionalization (slight amount of chemical modification) of SWCNTs forms luminescent sp3 carbon defects in the sp2 carbon lattice structures, producing locally functionalized SWCNTs (lf-SWCNTs) that are also called as organic color centers and luminescent quantum defects in SWCNTs [1-3]. Compared to original E
11 PL of pristine nanotubes, lf-SWCNTs show defect PL with red-shifted wavelengths and increased quantum yields from their defect sites. The chemical reactions used for the sp3 carbon formation in lf-SWCNTs are applicable to a technique for further functionalization such as the polymer gel coating layer formation around tube surfaces for their stable aqueous dispersibility [4] and the sensor function generation [5-10].

Regarding selective sensing system fabrication, molecular recognition at the defect sites of lf-SWCNTs is a promising method. Therein, the selective binding of target molecules at the defect sites of lf-SWCNTs via the molecular recognition induces specific spectral shifts and intensity changes of defect PL as sensor responses. For example, sugar and ion sensors are produced by introducing phenyl boronic acid and azacrown ether groups at the defect sites of lf-SWCNTs, respectively [5,6]. Moreover, lf-SWCNT sensors for protein detection/recognition are created based on unique microenvironment response of defect PL. Namely, an avidin-biotin interaction at the defect sites of lf-SWCNTs is used to form different microenvironments around the defect sites based on the selective adsorption of the avidin derivatives with different amino acid sequences, by which the avidin derivatives are differentiated from the defect PL shift responses [9]. Serum albumin (SA) sensor using lf-SWCNTs is fabricated by introducing a long-chain fatty acid group at the defect site. Therein, the strong interaction between SA and the fatty acid group induced selective defect PL spectral shifts. Moreover, various SAs from bovine, mice, and human (BSA, MSA, and HAS, respectively) are detected by this sensor. The lf-SWCNT sensor works in body fluids such as fetal bovine serum for BSA detection and albuminuria of diabetes mice for MSA detection [10].

Therefore, the molecular recognition systems in lf-SWCNTs have a high potential to develop advanced NIR sensors for biomolecule and protein detection/ recognition.

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Figure 1