Nanoscale & Nanoscale Advances Blog

SEM images of the polymer submicro-channels and photographs of the fabricated submicro-optofluidic PGMR filter with empty channels and with water filled channels, respectively.

Optical filters are routinely included in devices used for communications, displays and bio-sensing. One such class of optical filters, which have been frequently used, are guided-mode resonance (GMR) filters. Inherent errors with common GMR filters are assigned to fabrication difficulties and as a result, new classes of reconfigurable GMR filters have been created.

In this HOT article, Jin and co-workers have proposed and devised a novel polymer-based GMR (PGMR) which can be incorporated into lab on a chip devices and become tuned by the optical properties of the fluidic mixture present. The simple and low cost PGMR filter was fabricated in three stages, using two-beam interference lithography, floating nanofilm transfer, and finally, thermal bonding technology.

The tunability of this class of PGMR was tested by filling the fluidic channels with a range of liquid mixtures of differing refractive indices. The resulting PGMR exhibited high reflection efficiency in the visible wavelength region, a narrow band tuning range and a high tuning efficiency. The successful incorporation of the PGMR in an optofluidic device operating in the visible wavelength region opens up the possibility of the inclusion of such PGMR devices in future lab on a chip devices.

A tunable submicro-optofluidic polymer filter based on guided-mode resonance
Guohui Xiao, Qiangzhong Zhu, Yang Shen, Kezheng Li, Mingkai Liu, Qiandong Zhuang and Chongjun Jin
Nanoscale, 2015, 7, 3429-3434. DOI: 10.1039/C4NR07233B.

The last decade observed immense growth in applications of graphene, a carbon allotrope with applications in diverse areas of science, including fluorescent nanoparticles like quantum dots. However, the applications of graphene-based nanodevices in biological contexts have still been a bit slow. This is primarily due to the metal-free nature of graphene nanoparticles, leading to poor fluorescence abilities, and their lack of sensing abilities for various analytes.

Lin et al., in the current work, intelligently overcome these two problems by doping graphene quantum dots with nitrogen-containing species like amines, which not only make the dots much brighter, but also sensitive to various analytes. They show the detection of trinitrophenol, one of the most explosive materials in aqueous solutions, with good sensitivity and a detection limit of 300 nM. The stability of these doped dots to various acidic and alkaline conditions make them suitable for sensing applications in different samples and solutions, especially with biological samples, since the conditions are extremely diverse in these samples. The sensitive detection of trinitrophenol in solutions using simple fluorescence based assays can actually be explored for commercial low cost detectors.

Although much remains to be done to make these dots universally applicable, like doping with different molecules to sense specific analytes, the initial platform technology makes these nanoparticles suitable for applications in biosensing to detect materials like toxins, poisons or even explosives.

Dr Dhiraj Bhatia

A facile synthesis of highly luminescent nitrogen-doped graphene quantum dots for the detection of 2,4,6-trinitrophenol in aqueous solution
Liping Lin, Mingcong Rong, Sisi Lu, Xinhong Song, Yunxin Zhong, Jiawei Yan, Yiru Wang and Xi Chen
Nanoscale, 2015, 7, 1872-1878. DOI: 10.1039/C4NR06365A

Dr Dhiraj Bhatia is a guest web writer for the Nanoscale blog. He is a chemist by training and received his PhD in Chemical Biology of Nucleic Acids from the National Center for Biological Sciences, TIFR India with an outstanding thesis award in 2013. He joined the Chemical Biology department at the Curie Institute, Paris, as an HFSP long term Postdoctoral Fellow and is currently investigating the mechanisms of endocytois using various chemical biology tools.

Whilst undertaking my PhD I was repeatedly warned that the synthesis of nanoparticles was a ‘dark art’ i.e. luck. When, in fact, synthesising these particles actually turned out to be the easy part, whilst the most difficult parts were attempting to achieve stable nanoparticle systems and understanding their resulting properties.

Gold nanocrystals: description and experimental setup.

Even as the field of nanotechnology has undergone considerable expansion there still remain many unanswered fundamental questions. One such question is whether the mechanical and electrical properties of gold nanoparticles are representative of those of the bulk material and how these properties become modified in the presence of an organic coating layer.

Smaali and co-workers have tackled this issue by performing quantitative analysis on the mechanical and electrical properties of thiolated gold nanocrystals. Utilising a recently published procedure, single nanoparticles were grown in a silicon bed with an alkyl-thiol coating on one side and an ohmic contact on the other. To investigate the properties of these nanoparticles, a conducting atomic force microscope (AFM) tip was used, primarily as it offered the ability to interact with the nanocrystals using a variable loaded force with high precision, but also, due to the presence of an ohmic contact, the electrical properties of the nanocrystals and the alkyl-thiol coating could be tested.

Employing AFM measurements and finite elemental analysis (FEA) simulations, the Youngs Modulus of the embedded single nanocrystals was estimated to be 4 times smaller than those of free standing single nanocrystals. The molecular junctions formed between the thiol SAM coating and the individual nanoparticles provided an interesting insight into electronic properties of these materials, with a significant decrease in the tunnel current decay factor and potential barrier height being measured when forces even in the low nN range were applied. These results have been substantiated through FEA and DFT calculations to be a result of strain-induced molecular deformation, which causes a significant impact on the interfacial dipole, resulting in a change in the HOMO position with respect to the Au Fermi energy.

The authors of this study predict that this research could be used as a base model for other studies of functional molecular junctions and mechanical switches, with even small changes in van der Waals forces of a few nN thought to be sufficient to change the electron properties of nanoparticle-based molecular electronic devices.

On the mechanical and electronic properties of thiolated gold nanocrystals
K. Smaali, S. Desbief, G. Foti, T. Frederiksen, D. Sanchez-Portal, A. Arnau, J. P. Nys, P. Leclère, D. Vuillaume and N. Clément
Nanoscale, 2015, 7, 1809-1819. DOI: 10.1039/C4NR06180B

Dr Derek Craig is a guest web writer for the Nanoscale blog. He is a Post Doctoral Research Fellow at the University of St. Andrews based in the fields of Biophotonics and Materials Science. With a background in chemistry, his work mainly focuses on the synthesis of nano to meso materials and the use of imaging techniques to study biological samples.

Confined surface plasmon sensors based on strongly coupled disk-in-volcano arrays

Research concerning plasmonic materials has erupted in recent years due to the unique optical and electrical properties afforded by nanoscale structures. The optical properties of plasmonic materials can be tuned by altering the distance between the gaps of metallic nanostructures, which support surface plasmon resonances (SPR), on a patterned array. The gaps between the nanostructures produce significantly high electric field enhancements that enable the sensitive detection of biomolecules or chemicals within the gaps.

In this HOT article, Ai and co-workers have produced unique plasmonic substrates for sensing applications: disk-in-volcano arrays. The arrays were formed using colloidal lithography techniques wherein polystyrene spheres (PS) were first deposited onto a substrate coated with a photoresist film. Next, active ion etching (RIE) was used to etch the film followed by a vertical silver (Ag) deposition. Finally, the PS and photoresist film were removed with toluene and ethanol, followed by another Ag deposition. The final structures consist of a cavity containing a disk with small nanogaps between the disk and the wall of the “volcano”. The proximity of the disk to the wall results in plasmonic coupling and greatly enhanced electric fields.

The advantages of the confined sensing area of the disk-in-volcano arrays are low background, due to the decreased detection area, and low reagent consumption, making these arrays particularly cost effective. These favorable advantages mean disk-in-volcano arrays show potential in applications such as biosensing, optical and electrical trapping and single-molecule analysis.

Dr Lee Barrett is a guest web writer for the Nanoscale blog. Lee is currently a postdoctoral researcher in the Centre for Molecular Nanometrology at the University of Strathclyde. His research is currently focused on the development of nanoparticle-based sensors and surface enhanced Raman scattering (SERS).