Temporal Fluctuations in Single-Molecule SERS Spectra

Abstract

New advances in ultrasensitive spectroscopy, able to detect a single molecule (SM), are of utmost interest in different disciplines from both fundamental and application aspects [1, 2, 3]. Earlier SM fluorescence studies at ambient conditions, showed blinking, spectral jumps, intensity fluctuations, yielding thus new insight into phenomena usually wiped out in ensemble average [4, 5]. Although Raman spectroscopy is characterized by an extremely low cross section, SM detection can be reached when the molecules are adsorbed onto metallic surfaces with nanometer-scale roughness or onto metal nanoparticles [6, 7]. Such an approach, called surface-enhanced Raman spectroscopy (SERS), gives rise in a drastic increase of the Raman cross section (up to 10) [8, 9, 10], as due to two, likely cooperating, mechanisms: an electromagnetic (em) local field enhancement and a charge transfer (CT) between the molecule and the metal surface [6,7]. The latter, which requires a tight, chemical interaction between the molecule and the metal [11,12], has been recently suggested to play a dominant role in SM SERS spectra [13]. With respect to other SM spectroscopies, SERS is endowed with a high sensitivity coupled to a rewarding chemical and structural specificity [14]. Therefore, SERS is one of the most sensitive spectroscopic approaches available for analytical chemistry, nanomedicine and nanotechnology [15, 16]. SERS spectra in the SM regime exhibit drastic temporal fluctuations in both line intensity and frequency [17, 18]. The vibrational mode emission of a SM has been shown to undergo a characteristic intermittent behavior that might encode both the dynamics of the molecule and the details of its interaction with the environment [19]. Information about the structural conformations sampled by the molecule can also be obtained; this being particularly relevant for proteins, due to the complex energy landscape explored during their dynamical evolution [20]. Therefore, an investigation of the SM spectral fluctuations, based also on suitable statistical approaches, may help in addressing some fundamental issues, such as ergodicity, statistical aging, entanglement of vibrational modes, nonstationarity of the emission processes, etc. [21, 22, 23]; these being of high relevance for a detailed knowledge of the fundamental dynamical processes in