Abstract
Abstract Photothermal imaging is useful for detecting individual nanoparticles and obtaining the absorption spectra. This study presents a wide-field photothermal reflectance spectroscopy technique achieved by incorporating a pump beam, a probe beam, and a charge-coupled device (CCD) camera into a commercial microscopic setup. The presented design does not require precise alignment between the pump and the probe beams and enables the observation of numerous individual nanoparticles during image acquisition. Despite the use of a simple imaging processing method, i.e., a four-bucket method using a CCD camera, sufficient sensitivity for the spectral imaging of a single gold nanorod (20 nm diameter and 84 nm length) is demonstrated. Numerous individual nanoparticles within a wide field of view (240 μm × 180 μm) are detected in an image captures at an imaging measurement speed of 0.02 mm2 min−1. Furthermore, the proposed photothermal reflectance spectroscopy technique can detect the variation in the absorption peak of the measured spectra depending on the aspect ratio of individual nanoparticles within a spectral resolution of 1 nm.
Highlights
Single nanoparticle spectroscopy is an indispensable technology with biological, medical, and material science applications [1,2,3,4,5,6,7]
This study presents a wide-field photothermal reflectance spectroscopy technique achieved by incorporating a pump beam, a probe beam, and a charge-coupled device (CCD) camera into a commercial microscopic setup
The refractive index of the surrounding medium changed due to the heat generated from the gold nanorods (GNRs), and the reflected probe beam was collected by the CCD camera
Summary
Single nanoparticle spectroscopy is an indispensable technology with biological, medical, and material science applications [1,2,3,4,5,6,7]. Photothermal microscopy enables the detection of metal nanoparticles by monitoring the refractive index variations and is an important tool for implementing single nanoparticle absorption spectroscopy [15, 17,18,19,20,21,22]. Berciaud et al [17] developed the photothermal heterodyne imaging method, which involves the detection of changes in the refractive index around the nanoparticles (>1.4 nm diameter) generated by a timemodulated pump beam from the change in the frequencyshifted signal magnitude of a modulated probe beam. The proposed system has a simple configuration, which reduces the dependence on the optical axis alignment, and allows the detection of multiple nanoparticles during image acquisition It can record the spectra of individual nanoparticles with a sufficient sensitivity (20 nm diameter and 84 nm length GNRs) despite the simple imaging processing used, i.e., the fourbucket method. A dark-field microscopy system was used to directly distinguish nanoparticles from pollutants in the region of interest (ROI) of the sample before performing the photothermal experiment
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