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Single-molecule Raman spectroscopy of a cyanine dye in aqueous silver colloidal solution with the use of surface-enhanced Raman scattering at near-infrared excitation (NIR-SERS) is reported. A characteristic Poisson distribution of SERS signals due to the Brownian motion of single dye molecule-loaded silver particles reflects the probability of finding 0, 1, or 2 1,1'-diethyl-2,2'cyanine (PIC) molecules in the probed volume during an actual measurement and is evidence that single-molecule detection by SERS has been achieved. Spectra measured in 1 s collection time with 100 mW nonresonant 830 nm excitation provide a clear “fingerprint” of a single PIC molecule by showing its typical Raman lines between 700 and 1700 cm−1. Single-molecule Raman signals are also detected for the first time at the anti-Stokes side of the excitation laser. Effective Raman cross sections for PIC of ∼10−16 cm2 per molecule can be inferred from the ratio between “pumped” anti-Stokes and Stokes signals.

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Background-Lesion composition, rather than size or volume, determines whether an atherosclerotic plaque will progress, regress, or rupture, but current techniques cannot provide precise quantitative information about lesion composition. We have developed a technique to assess the pathological state of human coronary artery samples by quantifying their chemical composition with near-infrared Raman spectroscopy. Methods and Results-Coronary artery samples (nϭ165) obtained from explanted recipient hearts were illuminated with 830-nm infrared light. Raman spectra were collected from the tissue and processed to quantify the relative weights of cholesterol, cholesterol esters, triglycerides and phospholipids, and calcium salts in the examined artery location. The artery locations were then classified by a pathologist and grouped as either nonatherosclerotic tissue, noncalcified plaque, or calcified plaque. Nonatherosclerotic tissue, which included normal artery and intimal fibroplasia, contained an average of Ϸ4Ϯ3% cholesterol, whereas noncalcified plaques had Ϸ26Ϯ10% and calcified plaques Ϸ19Ϯ10% cholesterol in the noncalcified regions. The average relative weight of calcium salts was 1Ϯ2% in noncalcified plaques and 41Ϯ21% in calcified plaques. To make this quantitative chemical information clinically useful, we developed a diagnostic algorithm, based on a first set of 97 samples, that demonstrated a strong correlation of the relative weights of cholesterol and calcium salts with histological diagnoses of the same locations. This algorithm was then prospectively tested on a second set of 68 samples. The algorithm correctly classified 64 of these new samples, thus demonstrating the accuracy and robustness of the method. Conclusions-The pathological state of a given human coronary artery may be assessed by quantifying its chemical composition, which can be done rapidly with Raman spectroscopic techniques. When Raman spectra are obtained clinically via optical fibers, Raman spectroscopy may be useful in monitoring the progression and regression of atherosclerosis, predicting plaque rupture, and selecting proper therapeutic intervention. (Circulation. 1998;97:878-885.)