The researchers examined two commercially produced systems of methoxy polyethylene glycol-coated gold nanorods, identified by nominal resonance labels of 800 and 660 nm. Manufacturer-supplied transmission electron microscopy characterized the gold cores. The more elongated particles had an average length of 49.9 nm and width of 12.2 nm, while the shorter particles measured 31.0 nm in length and 13.3 nm in width. Their mean aspect ratios were represented in the optical calculations as 4.14 and 2.36, respectively. The longer, more elongated material displayed a dominant longitudinal absorption band near 800 nm; the less elongated material had its corresponding band near 642 nm. Both samples also exhibited smaller transverse features in the visible region, at approximately 511 and 515 nm.

This separation in optical response allowed each suspension to be irradiated close to its own strongest longitudinal plasmon resonance: an 820 nm pulsed laser for the nominal 800 nm sample and a 640 nm pulsed laser for the nominal 660 nm sample. The two sources generated pulses with widths of 6–39 ns and 5–39 ns, respectively, and offered internal repetition rates of 1, 5 and 10 MHz. Their maximum pulse energies were 3.5 nJ at 820 nm and 2.0 nJ at 640 nm. The reported laser power ranged from 1.7 to 32.7 mW for the 820 nm source and from 0.3 to 15.0 mW for the 640 nm source. Illumination lasted 10–50 seconds, with the gold nanorods dispersed in 0.5 cm³ of water in a sample holder having a 1 cm optical path length.

An infrared thermal camera with a 7.5–13.0 μm spectral range was positioned above the holder to track temperature during and after irradiation. At the maximum 820 nm power of 32.7 mW, the thermal image showed localized heating around the illuminated region, where the maximum temperature increase reached approximately 17 °C. The measured rise was about 3 °C at 1.7 mW, around 6 °C at 9.9 mW and approximately 8 °C at 15.7 mW. Temperature increased rapidly during illumination and then approached a plateau as heat generation and dissipation reached a balance. After the laser was switched off, the sample cooled toward ambient temperature. In the 640 nm experiments, the highest tested power of 15.0 mW produced a maximum rise of 9.5 °C, and the suspension returned to ambient levels in roughly 24 seconds in the reported observation.

Concentration was as influential as power in determining the bulk response. For the 820 nm experiments, temperature increase varied approximately linearly with laser power at each of three nanorod concentrations: 1.150, 0.575 and 0.375 mg/mL. The slope became progressively steeper as concentration increased, showing that more particles in the suspension produced a larger temperature change for a given laser power. At 32.7 mW, the 1.150 mg/mL suspension heated by approximately 17 °C, compared with about 9 °C at 0.575 mg/mL under the same conditions. The 640 nm series used concentrations of 1.840, 0.920, 0.460 and 0.230 mg/mL and also showed an approximately linear temperature-power relationship with the same overall concentration-dependent trend. The tested 820 nm system generated larger absolute temperature elevations, but the experiments did not isolate every factor responsible for the difference between the two setups.

The researchers fitted the heating and cooling records separately with biexponential functions because each curve contained two resolvable macroscopic timescales. For a representative 820 nm experiment conducted at 32.7 mW and 1.15 mg/mL, the fitted heating time constants were 0.20 and 3.08 seconds. The corresponding cooling constants were 0.76 and 10.65 seconds. Across all evaluated powers, fast time constants ranged from 0.18 to 0.83 seconds, while slow constants ranged from 3.08 to 13.55 seconds. Cooling generally had the longer slow component, consistent with heat continuing to diffuse through the bulk suspension and exchange with the cuvette and surrounding apparatus after illumination ended. For the 640 nm series, slow cooling constants occupied a comparatively narrow range of 9.02–10.51 seconds. These fits were restricted to power-dependent measurements at fixed concentrations because complete heating and cooling traces were not available for every concentration tested.

Neither the fast nor the slow fitted constants showed a consistent monotonic relationship with laser power at either wavelength. The slow heating constant generally became smaller at higher powers for the 820 nm series, although not in a strictly monotonic pattern, while the 640 nm heating constants varied non-monotonically. The slow cooling constants were comparatively less sensitive to power, especially at 640 nm. The results therefore separate two kinds of control: laser power primarily changed the amplitude of the temperature rise, whereas the effective thermal timescales were governed chiefly by heat capacity, thermal conductivity, experimental geometry and heat-loss conditions. The measured constants should not be interpreted as the intrinsic relaxation times of individual nanoparticles: electron-to-phonon and phonon-to-phonon processes occur on picosecond-to-nanosecond scales, many orders of magnitude faster than the seconds resolved by the infrared camera.

Transmission electron microscopy and a quasistatic Gans model provided an optical interpretation of the two nanorod populations. The model represented randomly oriented prolate gold ellipsoids, using a complex dielectric function derived from interpolated Johnson–Christy constants for gold. Longitudinal and transverse responses were weighted by one-third and two-thirds, respectively, and averaged over fixed Gaussian approximations to the aspect-ratio distributions. Because direct particle-by-particle histograms and PEG-shell properties were unavailable, these distributions were based on reported dimensional coefficients of variation, and the coating thickness, grafting density and hydration were not independently characterized. Effective surrounding relative dielectric constants of approximately 2.05 and 1.91 gave the best qualitative agreement for the nominal 660 and 800 nm samples. The calculation reproduced the principal resonance assignments but not every detail of the measured spectra, which were broader than their calculated counterparts.

The mPEG coating formed part of the tested system rather than a separable variable. It affected the local optical environment around the gold cores and introduced thermal resistance around each particle. Modeling cited by the authors predicted that the coating could increase the steady-state temperature rise by 25–30% and extend the equilibration timescale by 50–75% relative to uncoated nanorods, but those estimates were not independently verified in this experiment. The measured absorption and heating responses therefore characterized the complete coated suspension in water, not bare gold nanorods. Likewise, the optical model addressed the samples’ spectral responses; it did not connect their geometry directly to the observed heating curves. Together, the experiments show that power and concentration can tune the thermal output within the tested ranges, while the simplified optical model helps interpret the observed resonances. Translating these physical relationships into a cancer-treatment protocol would require evidence beyond nanoparticle suspensions, including direct assessment of cellular effects and an appropriate model of treatment conditions.

Subject of Research: Photothermal heating of gold nanorod suspensions by resonance-matched pulsed lasers

Article Title: Quantitative analysis of thermal dynamics in gold nanorods, laser and concentration effects with relevance to cancer therapy

Article References: Al Helou, N., Alwan, I., Yassine, H., Matar, M., Tfaili, J., Zibara, Z., Hussari, K., & Nicolas, R. (2026). Quantitative analysis of thermal dynamics in gold nanorods, laser and concentration effects with relevance to cancer therapy. Results in Optics, Article 101190. https://doi.org/10.1016/j.rio.2026.101190

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101190

Keywords: gold nanorods, photothermal heating, plasmon resonance, pulsed lasers, infrared thermography, nanoparticle concentration, heat transfer, thermal kinetics, biomedical optics, Quantitative, analysis, thermal