image: The system combines a common-path interferometer (TWINS) with a fast 2D camera. Dark-field scattering from gold nanoparticles is collected through a water-immersion objective while an electrochemical potential is applied. The wedge position in TWINS controls the time delay, and Fourier transformation of the resulting interferograms yields a full spectrum for every pixel in the field of view.
Credit: ©Science China Press
Single nanoparticles often show strikingly different electrochemical behaviors from one another due to variations in size, shape, surface defects, and local environment. However, conventional dark-field spectroscopy can only acquire a spectrum from one nanoparticle at a time, making statistically meaningful studies extremely time-consuming.
Now, a team led by Jin-Hui Zhong at Southern University of Science and Technology (SUSTech) has developed a wide-field Fourier transform hyperspectral imaging system that solves this throughput bottleneck. The method integrates a common-path interferometer called TWINS—which provides exceptional phase stability—with a 2D camera capable of 100 frames per second.
In this approach, the camera records interference patterns as a moving wedge in the interferometer varies the time delay between two replicas of the signal field. A simple Fourier transform of the resulting interferograms yields a full spectrum for every pixel in the field of view simultaneously—up to 2048 × 2048 spectra in a single data cube.
The team demonstrated the technique by monitoring electrochemical deposition of palladium (Pd) and cadmium sulfide (CdS) onto gold nanoparticles—a common route for preparing bimetallic or metal-semiconductor photocatalysts. In dark-field scattering mode, the plasmon resonance of each nanoparticle serves as a sensitive reporter of changes in size, shape, and local dielectric environment.
The results revealed striking heterogeneity. While most gold nanospheres showed a continuous red shift in their scattering peak as palladium deposited uniformly on their surfaces, gold nanorods exhibited a more complex sequence: an initial red shift as palladium preferentially deposited at the rod tips (increasing the aspect ratio), followed by a blue shift as deposition expanded to the sides, and finally another red shift when the particles became too large for the rod model to apply. Notably, about 3% of nanorods showed the opposite sequence—deposition initiating on the sides—highlighting particle-to-particle variations that would be missed in single-particle studies with limited statistics.
By plotting the rate of spectral shift against applied potential, the researchers derived "optical" voltammetry for individual nanoparticles. This optical analogue proved more sensitive than conventional electrochemical current measurements, detecting deposition onset at ~0.45 V—well before the electrochemical current showed a significant rise at ~0.40 V.
The system's modular design allows operation in multiple spectroscopic modes, including photoluminescence, transmission (absorption), and potentially Raman scattering. The researchers envision applications ranging from real-time control of electrocatalyst synthesis and high-throughput screening of catalyst libraries to monitoring structural evolution in battery materials during cycling.
Method of Research
Experimental study