UV light illuminates the tremendous sensing potential of single-walled carbon nanotubes
(Nanowerk Spotlight) Single-walled carbon nanotubes (SWCNTs), with their diameter of about 1 nm, are directly comparable to the size of single biomolecules and to the electrostatic screening length in physiological solutions. SWCNTs arguably are the ultimate biosensor among nanoscale semiconducting materials due to their high surface-to-volume ratio and unique electronic structure. Furthermore, a SWCNT consists solely of surface so that every single carbon atom is in direct contact with the environment, allowing optimal interaction with nearby biomolecules (read more: "Biosensing mechanism with carbon nanotubes explained").
"After more than a decade of excitement, more and more researchers in the nanotube field believe that pristine SWCNTs are very limited as a sensing material," Dr. Gugang Chen, a Senior Scientist at the Honda Research Institute USA (HRI), tells Nanowerk. "Ironically the ultrahigh sensitivity of SWCNTs is easily compromised by various unintentional contaminants from the device fabrication process as well as the ambient environment. As a result, significant efforts have been focused on all kinds of ways to functionalize or decorate nanotubes with other species in order to improve their sensitivity."
In a paper in the March 29, 2012 online edition of Scientific Reports ("Enhanced gas sensing in pristine carbon nanotubes under continuous ultraviolet light illumination"; open access), Chen and his HRI colleagues led by Chief Scientist Dr. Avetik R. Harutyunyan, show that applying continuous in situ ultraviolet (UV) light illumination during gas detection can enhance a SWCNT-sensor's performance by orders of magnitude under otherwise identical sensing conditions. The research was managed by Project Director Jun Sasahara.
These results indicate that, despite significant progress that has been made in the past decade, what had been achieved before was still far from what a pristine SWCNT-based sensor can truly offer. In addition, although the device cleaning with UV light irradiation on nanotube conductance is not new, the degree of impact in the case of continuous in situ cleaning has surprised the scientist.
This illustration captures the essence of the work. (Image: Dr. Chen, HRI)
For their experiments, the HRI team first investigated the effect of UV light illumination on a SWCNT sensor's performance in dry air to get a reference value. Then they exposed the sensor to nitrogen monoxide (NO) and recorded decreasing responses with subsequent exposures.
"We attributed the smaller response at later cycles to partial device recovery which appears to have many active sites still occupied by pre-adsorbed NO molecules," explains Chen. "The assumption has been justified when we found that the application of UV light illumination during air flushing not only dramatically reduced the recovery time down to a few seconds but also enhanced the signal 5 times. This observation prompted us to investigate the idea of in situ device cleaning during the process of gas detection."
While the in situ device cleaning during sensor operation indeed leads to a significant sensitivity enhancement, the researchers also found that the sensor would eventually lose its performance in the presence of oxygen.
"We observed that the life time of the sensor depends on the thickness of the SWCNT film, e.g., thicker SWCNT films appeared to last longer," says Chen. "An electrical study showed that the SWCNT sensor continuously loses conductance under UV light illumination. Our observation suggests that SWCNTs are probably gradually removed by the UV light irradiation – something that requires further investigation in order to fully understand the detailed mechanism of this removal."
In order to prevent damage to the SWCNTs, the team then studied the effect of UV light illumination in a controlled inert environment (flowing nitrogen). Here they found that the UV light doesn't do any noticeable damage to nanotubes.
Chen notes that there are two important findings in the current work: "Firstly, our results demonstrate that surface cleanness is of paramount importance to a nanosensor's performance. The best gas detection levels that we have achieved under the same sensing conditions except surface cleaning with in situ UV light are 3 to 5 orders of magnitude worse."
"Secondly" he continues, "the ultrahigh gas sensitivity that we have achieved with pristine single-walled carbon nanotubes constitutes a significant advance in the field. For example, for nitric oxide detection, our study clearly demonstrated sensing at 10 parts-per-trillion (ppt) with detection limit down to 590 parts-per-quadrillion (ppq) at room temperature, which is 10,000 times better than current state-of-the-art results. Gas detections on NO2 and NH3 also showed sensitivities at least 2 to 3 orders of magnitude better than what previously had achieved. For comparison, the most sensitive level of NO2 detection was reported to be in a few tens of ppt with a laser-based instrument system, a comparable level where a specially trained dog can detect individual species."
Chen points out that his team's experiments and observations indicate that SWCNTs are so sensitive to their environment that any minor imperfection of the sample sealing system or very low level of interactive impurities present in a 99.9999% pure inert carrier gas (nitrogen or argon) will be enough to dope nanotubes and make their conductance increase after the UV light is turned off.
"These results further point to the intrinsic ultrasensitivity of pristine single-walled carbon nanotubes and the crucial role of surface cleanness prior to molecular sensing," he says.
At the nanoscale, these findings become very critical because any nanosensor is inevitably compromised by unintentional contaminants if the issue is not addressed actively.
The researchers suggest that, because of its simplicity, their approach can easily be deployed in existing sensor architectures, with possibly broad implication in the nanosensor field.
"What we were able to demonstrate is only a beginning" says Chen. "Future challenges for us include how to achieve better selectivity on a SWCNT-based sensor; protection of the sensing material in a practical detection environment from possible damage caused by cleaning; and how to incorporate in situ cleaning such as UV light into the sensor platform."