Monitoring the Transient Thermal Infrared Emission of Gold Nanoparticles upon Photoexcitation with a Step-Scan Fourier-Transform Spectrometer

The transient photothermal process of gold nanoparticles (AuNP) capped with different molecules, namely, citrate, cetyltrimethylammonium bromide (CTAB), and methoxyl-polyethylene glycol thiol (mPEG), has been investigated by time-resolved infrared emission spectroscopy monitored with a step-scan Fourier-transform spectrometer. Upon photoexcitation of the surface plasmonic resonance band of AuNPs with a 532 nm nanosecond pulsed laser, the transient infrared emission was observed within about 1 μs, referring to the duration of the laser heating and thermalization of AuNPs. Comparing the infrared emission contours with the blackbody radiation spectra at different temperatures revealed that the temperature reached 400 ± 100 °C in 90–120 ns as the 24 nm mPEG-capped AuNPs were excited by a peak power of 5 × 10¹⁰ W m^–2 (25 mJ cm^–2 from a 5 ns pulsed laser) at 532 nm. The insignificant changes in the morphology and size distribution of mPEG-AuNP suggested that the surface modification via covalent bonding helped retention of the morphology of the nanostructures after laser heating. In addition, photoexcitation of 35 nm CTAB-AuNPs generated a higher transient temperature than that of 89 nm CTAB-AuNP; this is consistent with the prediction by Mie theory that smaller nanoparticles possess a higher contribution of absorption in the extinction coefficient, which leads to higher photothermal efficiency. This is the first time that the transient broadband thermal infrared emission of the photoexcited gold nanoparticles has been recorded within a submicrosecond, which is close to the nascent condition. The duplexity in the temporal capability and broadband spectroscopic window of the time-resolved emission infrared spectroscopy provides a promising noncontact thermometer to illustrate the photothermal process and quantify the transient temperatures of miscellaneous metallic nanostructures upon photoexcitation