Ultra-low Noise Superconducting Nanowire Single-photon Detector (SSPD) for Fluorescence Correlation Spectroscopy (FCS) toward Early Diagnosis of Alzheimer Disease

December 22, 2015

National Institute of Information and Communications Technology
Hokkaido University
Osaka University

SSPD photon detector has enabled FCS to measure rotational diffusion.
Measuring rotational diffusion will detect a very early stage of protein aggregates.
Detecting small protein aggregates will lead to early diagnosis of Alzheimer disease.

As a collaboration of NICT, Hokkaido University and Osaka University, we succeeded in developing a method to measure rotational diffusion of fluorescent molecules in solution. This was accomplished by fluorescence correlation spectroscopy (FCS) using a superconductive nanowire single-photon detector (SSPD). A conventional FCS method requires two sets of photon detectors to eliminate after-pulse noises. The SSPD is free of after-pulse noises and thus enables measuring rotational diffusion with a single detector. Using this method, we now can obtain higher signal-to-noise ratios compared with conventional methods. The capability of measuring rotational diffusion of protein molecules makes it possible to detect a very early stage of protein aggregates, which are causes of neuronal disorders, such as Alzheimer and prion disease. Thus, an outcome of this study leads to clinical applications of the SSPD. The results of this study were published in an American journal “Optics Express” on December 14, 2015.

Background

Fluorescence correlation spectroscopy (FCS) is well known method to determine the translational diffusion coefficient and the concentrations of fluorescence-tagged target molecules within a cell. By employing polarization-dependent FCS (pol-FCS) method, we can estimate the rotational diffusion coefficient, which is very sensitive to the molecular size. However, the relaxation time of biomolecular rotational diffusion is usually in the sub-microsecond range, which is comparable to the time range of the after-pulse noise of photo detectors, such as avalanche photodiodes (APDs).

NICT has developed a superconducting single-photon detector (SSPD) for quantum communication systems and succeeded in demonstrating the system detection efficiency (SDE) of 80% at 1550 nm [1]. SSPD has not only high SDE but also after-pulse-free nature, which is very promising for FCS application. NICT has developed visible-wavelength SSPD (VW-SSPD) for FCS system in collaboration with Hokkaido University and Osaka University since 2013 and achieved the SDE of 76% at 635 nm [2,3].

Achievements

We succeeded in the measurement of the rotational diffusions of CdSe/CdS (core/shell) Qrod, a kind of quantum dot with a rod-like shape with the diameter and length of 7 nm and 22 nm, by using pol-FCS with VW-SSPD. The signals from rotational diffusion were never detected by single APD due to after-pulse noise. The shape of Qrod estimated from the relaxation time of observed rotational diffusion almost agreed with that estimated from transmission electron microscope (TEM) observation, suggesting that the pol-FCS with VW-SSPD is expected as a new tool for measurements of the small changes of the volume such as early stage of protein aggregates.

Future perspectives

The pol-FCS with VW-SSPD is very effective for early diagnosis of neurodegenerative disease such as Alzheimer. Toward clinical applications of the SSPD, we will improve the performance of VW-SSPD to realize the pol-FCS system with higher sensitivity for the molecular size.

Appendix

Fig. 1 Measurement principle of fluorescence correlation spectroscopy (FCS).

A) Fast moving of small size molecule in detection area (Confocal volume), and fast fluctuation of fluorescence intensity, (B) Slow moving of large size molecule in detection area (Confocal volume), and slow fluctuation of fluorescence intensity, (C) The relationship between fluorescence autocorrelation function and size of molecule.

Fig. 2 Visible-wavelength SSPD optimized for the pol-FCS system.

In VW-SSPD, multi-layered dielectric mirror, which is designed to have the reflectance of more than 99% at visible wavelength, was adopted and photons are illuminated from the top of Si substrate, while photons are illuminated from the backside in SSPD for telecom wavelength (1550 nm) (Fig.2 (a)).

We also adopted a circular detection area with a diameter of 35 μm to achieve effective coupling with multi-mode optical fiber with a core diameter of 50 μm (Fig.2 (b)). The graded-index (GRIN) lens are attached at the end of fiber to focus the photon spot to the detection area. As a result, we have achieved the system detection efficiency (SDE) of 76% at 635 nm (Fig.2 (c)).

Fig. 3 Measurement setup and schematic diagram of principles of polarization-dependent FCS(pol-FC).

Fig.3 (A): Fluctuating fluorescence intensity due to random crossing of fluorescence molecules over confocal volume (translational diffusion).

Fig.3 (B): Fluorescence molecules randomly rotating (rotational diffusion) during translational diffusion, and the polarization of fluorescence is also rotating corresponding to the rotational diffusion. When such the fluorescence is observed through an analyzer, the fluorescence intensity fluctuates by not only translational diffusion but also rotational diffusion.

Fig. 3 (C): Generally, the rotational component of auto-correlation function decays faster than that of the translational diffusion because the rotational diffusion is much faster than the translational diffusion. The molecular shape estimated by fitting results of the autocorrelation functions was corresponding to the shape of Qrod. Thus, it was validated that the component obtained from the pol-FCS using SSPD is the rotational diffusion component.

Fig. 4 Typical results of pol-FCS using (A) APD and (B) SSPD.

In the case of APD, strong noise component due to after-pulse (blue arrow) disturbed the rotational diffusion component (red arrow). In contrast, noise-free rotational diffusion component was observed using SSPD.

Acknowledgements

This study was supported in part by the Japan Science and Technology Agency (JST) and the Japan Agency of Medical Research and Development (AMED).

References