Temperature-Induced Luminescence Intensity Fluctuation of Protein-Protected Copper Nanoclusters: Role of Scaffold Conformation vs Nonradiative Transition

Abstract

Protein-scaffolded atomically precise metal nanoclusters (NCs) have emerged as a promising class of biofriendly nanoprobes at the forefront of modern research, particularly in the area of sensing. The photoluminescence (PL) intensity of several nanoclusters showed a systematic temperature-dependent fluctuation, but the mechanism remains ambiguous and is poorly understood. We tried to shed some light on this mechanistic aspect by testing a couple of hypotheses: (i) conformational fluctuation of the protein scaffold-mediated PL intensity fluctuation and (ii) PL intensity fluctuation due to the variation in the radiative and nonradiative transition rates. Herein, the PL intensity of the lysozyme-capped copper nanocluster (Lys-Cu NC) showed excellent temperature dependency; upon increasing the temperature, the PL intensity gradually decreased. However, contrasting effects can be seen when the nanocluster is exposed to a chemical denaturant (guanidine hydrochloride (GdnHCl)); the PL intensity increased with the increase in the GdnHCl concentration due to the change in the ionic strength of the medium. This discrepancy clearly suggests that the thermal PL intensity fluctuation cannot be explained by a change in the scaffold conformation. Furthermore, upon closer investigation, we observed a 2-fold increase in the nonradiative decay rate of the Lys-Cu NC at the elevated temperature, which could reasonably explain the decrease in the PL intensity of the nanocluster at the higher temperature. Additionally, from the result, it was evident that the protein scaffold-metal core interaction played a key role here in stabilizing each other; hence, the scaffold structure remained unaffected even in the presence of chemical denaturants.

Figure 1

Temperature-dependent systematic PL intensity variation for Lys-Cu NC (λ_ex = 365 nm) (A) ascending from 20 to 60 °C and (B) descending from 60 to 20 °C.

Figure 2

(A) Emission spectra of Lys-Cu NC (λ_ex = 365 nm) in the presence of increasing concentrations (0–6 M) of GdnHCl. (B) Ratio of PL intensities of Lys-Cu NC (λ_ex = 365 nm) (gray) and Trp-Cu NC (λ_ex = 380 nm) (blue) in the presence (F) and absence (F₀) of GdnHCl.

Figure 3

Emission spectra of Lys-Cu NC (λ_ex = 365 nm) in the absence (blue) and presence of 6 M NaCl (red). The inset shows a comparison of the change in PL intensities of the Lys-Cu NC upon the addition of 6 M GdnHCl or 6 M NaCl.

Figure 4

(A) Emission spectra of pure lysozyme (λ_ex = 280 nm) in the presence of increasing concentrations (0–6 M) of GdnHCl. (B) Ratio of PL intensities of pure lysozyme (λ_ex = 280 nm) (gray) and Lys-Cu NC (λ_ex = 280 nm) (blue) in the presence (F) and absence (F₀) of GdnHCl.

Figure 5

Time-resolved PL intensity decay profile (λ_ex = 280 nm; λ_em = 340 nm) of the pure lysozyme in the absence (red) and presence (cyan) of 6 M GdnHCl and Lys-Cu NC in the absence (blue) and presence (green) of 6 M GdnHCl. The black curve represents the instrument response function (IRF).

Figure 6

(A) Time-resolved PL intensity decay profile (λ_ex = 340 nm and λ_em = 437 nm) of Lys-Cu NC at 25 °C (blue) and 50 °C (red). (B) Temperature-dependent variation of the PL lifetime and quantum yield of the Lys-Cu NC.