Quantum confined peptide assemblies with tunable visible to near-infrared spectral range

Abstract

Quantum confined materials have been extensively studied for photoluminescent applications. Due to intrinsic limitations of low biocompatibility and challenging modulation, the utilization of conventional inorganic quantum confined photoluminescent materials in bio-imaging and bio-machine interface faces critical restrictions. Here, we present aromatic cyclo-dipeptides that dimerize into quantum dots, which serve as building blocks to further self-assemble into quantum confined supramolecular structures with diverse morphologies and photoluminescence properties. Especially, the emission can be tuned from the visible region to the near-infrared region (420 nm to 820 nm) by modulating the self-assembly process. Moreover, no obvious cytotoxic effect is observed for these nanostructures, and their utilization for in vivo imaging and as phosphors for light-emitting diodes is demonstrated. The data reveal that the morphologies and optical properties of the aromatic cyclo-dipeptide self-assemblies can be tuned, making them potential candidates for supramolecular quantum confined materials providing biocompatible alternatives for broad biomedical and opto-electric applications.

Enhanced Fluorescence for Bioassembly by Environment‐Switching Doping of Metal Ions

“…Fourier‐transform infrared spectroscopy (FTIR) analysis indicated that after Zn(II) coordination, the NH stretching vibration peaks of the cyclo‐HH became narrower and sharper, indicating that less hydrogen bonds were formed and many NH moieties remained free. In addition, the insert backbone CC stretching vibration peaks became relatively stronger, indirectly confirming that the active nitrogen atoms extensively participated in complexation with Zn(II) and the coordination distinctly hindered the IR absorption of the relevant chemical bonds . This confirmed that the active nitrogen atoms extensively participated in complexation with Zn(II) (Figure S8, Supporting Information), as we previously demonstrated for cyclo‐WW + Zn(II) . ]…”

“…Simultaneously, the photoluminescence efficiency was improved, with an enhanced QY of 15% for cyclo‐HH + Zn(II) (Table ). The emission of the cyclo‐HY solution was also enhanced to some extent in the presence of Zn(II), with the λ Em detected at 490 nm (Ex: 380 nm) (Figure S5, Supporting Information), τ of 4.79 ns and a QY of 9% (Table ), similarly to cyclo‐WW, as we have previously published . In contrast, the other dipeptides did not show any noticeable changes in the presence of Zn(II) (Figures S6 and S7, Supporting Information).…”

“…Briefly, after dissolving the cyclodipeptide powder in methanol and heating to 80 °C, most peptides were dissolved (except cyclo‐FF and cyclo‐YY). Atomic force microscopy (AFM) analysis demonstrated that the cyclodipeptides self‐assembled into diverse architectures with dimensions ranging from dozens to hundreds of nanometers, including nanospheres (cyclo‐WW), nanofibers (cyclo‐HH, cyclo‐FF, cyclo‐HY, cyclo‐WH, cyclo‐WY, cyclo‐HF, cyclo‐FY) and platelets (cyclo‐YY, cyclo‐FW) ( Table 1 ; Figure S2, Supporting Information). This implies that the supramolecular morphologies can be finely modulated by simply modifying the peptide residues .…”

“…Except cyclo‐YY, cyclo‐FF (with no detectable emissions) and cyclo‐HY (λ Em at 390 nm), all other cyclo‐dipeptide self‐assemblies showed fluorescence of no less than 400 nm (Table ). Notably, cyclo‐HH, cyclo‐WW and cyclo‐WH self‐assemblies showed two λ Em each, namely 425 nm (Ex: 340 nm) & 520 nm (Ex: 420 nm), 440 nm & 520 nm (Ex: 370 nm), and 380 nm (Ex: 320 nm) & 465 nm (Ex: 370 nm), respectively (Table , Figure S4A,C,F, Supporting Information). These spectroscopic findings indicated the hierarchical organization and the size‐encoded photoluminescence of the assemblies, consistent with the AFM results.…”

“…Herein, we extensively studied the self‐assembly and corresponding photoluminescent properties of all 10 cyclodipeptides composed of the natural aromatic amino acids, including histidine (H), W, F, and tyrosine (Y) (Figure S1, Supporting Information), and studied the coordination mechanism between zinc ions and cyclo‐HH, which shows the most effective doping consequence and photoluminescent enhancement. We demonstrate for the first time that following doping with Zn(II), the metal ions are initially attracted and pulled from the solvent to a more peptide‐rich environment, as opposed to the currently accepted notion stating that the two entities directly assemble in the solvent, thus giving rise to an “environment‐switching” doping mechanism.…”