Solid-State Dendrimer Sensors: Probing the Diffusion of an Explosive Analogue using Neutron Reflectometry and in-situ Photoluminescence Spectroscopy

Authors

Hamish Cavaye, Arthur Smith, Paul Burn, Ian Gentle, S.-C. Lo and Paul Meredith (University of Queensland), Michael James and Andrew Nelson (ANSTO)

Introduction

Oxidative photoluminescence quenching utilising conjugated polymers as the sensing material has proven to be one of the best methods to date for sensing explosive analytes. 

There are however a number of issues that make conjugated polymers difficult to work with, including complex morphologies, reproducibility of syntheses, polydispersity and the need to make relatively elaborate structures to reduce the close packing of chains. Most sensing has used very thin polymer films (of order <10 nm) to achieve a rapid response; although such films have been shown to have a lower photoluminescence intensity than thicker films.

For oxidative luminescence quenching to work efficiently the sensing material needs to be highly luminescent in the solid-state, have an affinity for the analyte, and have a lowest unoccupied molecular orbital (LUMO) energy sufficiently higher than the LUMO of the analyte.

In the absence of an analyte, photoexcitation of a luminescent conjugated dendrimer leads to an exciton that can decay radiatively; however, in the presence of an analyte the excited electron of the exciton is transferred to the LUMO of the analyte and the photoluminescence is quenched.

In this work (recently published in Langmuir) we report the first rapid reversible vapour-phase detection of the explosive analogue p-nitrotoluene using a fluorescent thin film of a first-generation dendrimer. Light-emitting dendrimers are a relatively new class of fluorescent material that have been successfully developed for use in light-emitting diodes.

They consist of a core, dendrons, and surface groups with the latter being primarily responsible for their processability into thin films. We show that a first generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9',9'-tetra-n-propyl-2,2'-bifluorene core (Figure 1) can detect a high electron affinity analyte.

Figure 1: First generation dendrimer comprised of 2-ethylhexyloxy surface groups

Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (23 nm) and thick (75 nm) films of the above dendrimer can rapidly and reversibly detect the explosive analogue p-nitrotoluene by oxidative luminescence quenching. 

For both the thin and thick films the photoluminescence is quenched by p-nitrotoluene by approximately 90% in 4 seconds, which is much faster than that reported for luminescent polymer films (Figure 2).

 
Figure 2. Photoluminescence in pristine dendrimer film and quenching of photoluminescence upon exposure to p-nitrotoluene analyte.

Combined photoluminescence and neutron reflectometry measurements using the Platypus time-of-flight neutron reflectometer (Figure 3) on pristine and analyte-saturated films gave important insight into the analyte adsorption processes.  It was found that during the adsorption process the photoluminescence was completely quenched and the dendrimer films swelled, being on average 4% thicker. 

In addition, by using deuterated p-nitrotoluene and protonated dendrimer films we were able to determine the adsorption profile of the analyte within these quenched films (Figure 4). On removal of the analyte by blowing N2 gas over the surface, the films returned to their original thickness and scattering length density and the Photoluminescence was restored showing that the sensing process was fully reversible.

Figure 3: Sample cell and photoluminescence spectrometer on the Platypus neutron reflectometer

 
Figure 4: Scattering Length Density profiles for pristine dendrimer film, the quenched film after exposure to the p-nitrotoluene analyte, and the recovered film after being purged by N2 gas.

Reference

1.  H. Cavaye, A. Smith, M. James, A. Nelson, P. L. Burn, I. R. Gentle and P. Meredith, “Solid-state dendrimer sensors: probing the diffusion of an explosive analogue using neutron reflectometry”, Langmuir 25, 12800-12805 (2009).