Redistribution of fluorescent molecules at the solid/liquid interface with total internal reflection illumination
Graphical abstract
Introduction
Development of new techniques for the manipulation of nanoscopic objects has aroused ongoing interest in diverse areas [1], [2], [3], [4]. Owing to the non-invasive remote manipulation feature as well as high tempo-spatial resolution capability, optical-based approaches enable previously inaccessible information to be extracted from chemo-physical systems and have become broadly useful [1], [2], [5], [6]. For example, optical tweezers are excellent tools to manipulate nanoparticles ranging in size from several micrometers to tens of nanometers (~25 nm to 10 µm) [7], [8]. With plasmonic dipole antennas, it is also possible to trap single gold nanoparticles with size down to 10 nm [9]. These techniques mainly take advantage of near field trapping forces in a tiny window to overcome the thermal motion. It is therefore very difficult to handle large number of objects simultaneously in a high throughput manner. In addition, strong laser intensity is commonly required (e.g., the required laser power for optical tweezers to trap a one hundred nanometer latex particle is normally as high as 5×106 W/cm2) [7]. This requirement is a major challenge for applications in biological systems that are sensitive to laser-induced thermal effect.
The rapid development of nano-fabrication techniques enables integration of novel nanostructures and optical microfluidic elements into a single device [3], [10], [11]. It therefore provides a promising approach to manipulate target objects in a high throughput fashion. This achievement is significant because it affords new insights into the development of novel separation methodology with the capability of regulating the selection process in a controllable way [12], [13]. However, some of those approaches are still dependent on the near field confinement effect and thereby, limits the strength of the trap and excludes the use of moderate laser power.
In many separation modalities, the dynamic adsorption and desorption processes of analytes on the matrix or channel wall constitute the basic selection mechanism during the separation process [14], [15], [16], [17], [18], [19], [20], [21]. Controlling the dynamic behavior of target objects at the interface in a chemo-physical manner is thus fundamentally important. It not only affords insightful information for the accurate elucidation of the separation mechanism but also provides new methodologies to manipulate target molecules in complex surroundings.
In this work, we demonstrate that the dynamic behavior of single Rhodamine B molecules close to a solid/liquid interface can be manipulated in a photo-induced route. Laser-induced repulsion enables regulation of the distribution of Rhodamine B molecules normal to the solid/liquid interface in a range of several hundred nanometers. To understand the mechanism in detail, comprehensive control experiments were performed to confirm this scenario. The laser-induced desorption kinetic curve is also well consistent with the simplified model raised according to the assumption as noted above. This observation is fundamentally interesting because moderate laser intensity (143 W/cm2) is sufficient to initiate this repulsion effect, potentially suitable for the development of photo-modulation technique with high throughput capability.
Section snippets
Experimental section
The single molecule imaging experiments were performed on a home-built prism type total internal reflection fluorescence imaging setup. In brief, the focused excitation light (with a final spot size on the sample around 2800 µm2) from a 532 nm diode laser was reflected to the side face of an isosceles triangle prism (IB-21.6–60.6-SF10, 25×25×25 mm, CVI Melles Griot, U.S.A.). In order to selectively record the fluorescent molecules close to the liquid-solid interface, the entrance angle of the
Redistribution of single molecules at the solid/liquid interface
In this study, clean cover slips (22×22 mm2) were firstly treated in an oven for 3 h at 150 °C. After this treatment, the adsorbed thin water layer on the surface was effectively evaporated and the silica surface became relatively hydrophobic even after dipping in water for one day. 7 μL of Rhodamine B solution (1 nM in deionized water) was evenly spread out between two cover slips. The depth of the sample solution is around 14.5 µm (determined by dividing the volume of the sample solution by the
Conclusion
In summary, in this study, it was found that long-range interaction at liquid-solid interface could be modulated in a photo-driven way on a hydrophobic-treated silica surface. This kind of interaction is fundamentally significant because it provides a non-invasive way to manipulate the dynamics of single molecules within a long distance at liquid-solid interface. This concept can be potentially extended to develop novel separation modalities such as photo-modulated chromatography (e.g.
Acknowledgements
This work was supported by NSFC (21405045, 21522502) Program for New Century Excellent Talents in University (China, NCET-13–0789) and Hunan Province Natural Science Funds for Distinguished Young Scholar (14JJ1017).
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