Poly(N-Isopropylacrylamide) Microparticles Produced by Radiation Pressure of a Focused Laser Beam:  A Structural Analysis by Confocal Raman Microspectroscopy Combined with a Laser-Trapping Technique

Abstract

We developed a confocal Raman microspectroscopy system combined with a laser trapping technique and applied it to aqueous solutions (H(2)O and D(2)O) of poly(N-isopropylacrylamide) (PNIPA), which is well-known as a representative thermo-responsive polymer, i.e., phase transition/separation between coiled and globular states. By introducing a near-infrared (1064 nm) laser beam into a microscope, PNIPA microparticles were produced at the focused spot of the laser beam, both in H(2)O and D(2)O. By using the present system, we succeeded in obtaining the Raman spectra of PNIPA in the coiled and globular states over a wide wavenumber region (800-3500 cm(-1)) for the first time. For the D(2)O solutions (in which the photothermal effect is negligible and hence the microparticles should be produced purely by the effect of radiation pressure), some significant differences were observed in the Raman spectra for the coiled state, in the globular state, and for laser induced microparticles. By analyzing these spectra in detail, we revealed that the structure of the laser-induced microparticles was analogous to that in the globular state. We also discuss the fundamental mechanism underlying the transformation of the higher order structure of a polymer by radiation pressure.

Insight into plasmonic hot-electron transfer and plasmon molecular drive: new dimensions in energy conversion and nanofabrication

“…Previously, microscopy observations revealed that the phase transition of PNIPAM resulted in microparticle formation when aqueous solutions were heated directly with a focused infrared laser. 83,84 This was also observed in D 2 O, for which the absorption of infrared light is negligible; the PNIPAM formed microparticles in response to the optical force. 83,84 Figure 19a shows the single-particle Rayleigh scattering spectral changes originating from the heating of a Au NP in 1.0% aqueous PNIPAM solution.…”

“…83,84 This was also observed in D 2 O, for which the absorption of infrared light is negligible; the PNIPAM formed microparticles in response to the optical force. 83,84 Figure 19a shows the single-particle Rayleigh scattering spectral changes originating from the heating of a Au NP in 1.0% aqueous PNIPAM solution. At a laser peak power density of 1.8 × 10 4 W cm − 2 (0.18 mW μm − 2 ), the spectral shift was within experimental error, whereas appreciable redshifts at 3.5 × 10 4 W cm − 2 and 5.3 × 10 4 W cm − 2 were recorded.…”

Polymer Channel Chips as Versatile Tools in Microchemistry

“…Raman spectroscopy/imaging can be also conducted by an analogous experimental setup. 19,20 By using a picosecond laser as an incident light source and a single photon counting system as a detector, 21 one can also perform picosecond time-resolved emission spectroscopy by using a microchannel chip. 15 Besides our experimental approaches, Kitamori et al have reported thermal lens spectroscopy for in situ observations of the phenomena proceeding in glass microchannel chips.…”

Photon Force-Induced Phase Transition Dynamics of Single Hydrogel Nanoparticles in Water

“…We have chosen nanoparticle suspensions as a model for polymer solution, and revealed that larger clusters of nanoparticles were first formed in a focal spot and evolved to a large assembly [31,32]. In PNIPAM assemblies, several studies by using fluorescence analysis and confocal Raman spectroscopy have presented that conformation of the polymer assemblies were strongly influenced by trapping laser power, supporting that the volume phase transition of PNIPAM assemblies can be induced by optical trapping [33]. Furthermore, not only in PNIPAM assemblies but also in the gels, µm sized polyacrylamide gels containing triphenyl-methane leuco derivatives show photoinduced rapid swelling responses [34].…”