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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SPARCLEs (Self-assembled Plasmonic Aggregates for Raman sensing and Catalysis in Liquid Environments)

Teaser

Summary

Metal nanoparticles, when assembled into higher order nanostructures, exhibit unusual optical properties which lend themselves to exciting applications for society such as plasmon-enhanced solar light harvesting (e.g. for solar cells), ultrasensitive chemical and biological sensing (e.g. for early detection of diseases), photocatalysis and optical circuitry (e.g. for faster communication). The remarkable optical properties of nanoparticles that give rise to these applications are dependent on nanoparticle size and shape, but are mostly governed by the spacing between nanoparticles. Whilst synthetic routes for controlling nanoparticle size and shape have dramatically improved during the past two decades, the development of methods for controlling inter-particle spacing has remained a fundamental scientific challenge.

Cucurbiturils (CB[n]s) are a family of barrel-shaped molecules. To date, CB[n]s with n=5-10, have been isolated and characterised. Prof. Scherman and Prof. Baumberg have previously demonstrated that CB[n] macrocycles can be utilised in producing photonic nanoarchitectures, forming rigid linkers between the nanoparticles providing accurate interparticle spacings of 0.9 nm (the thickness of a CB[n] molecule). Spacings of 0.9 nm are within the `close-coupling regimeâ€™ for plasmonic structures, where focussing of the incident electromagnetic radiation between nanoparticles is most intense (termed `hot-spotsâ€™). Moreover, these molecules are capable of accepting guest molecules into their internal cavity. While CB[5]-CB[7] can accommodate one guest, the larger homologue CB[8] can even accommodate two guests. This is an extremely useful trait for ultrasensitive sensing (detecting and studying molecules).

Nanoparticles and assembled nanostructures show promise in a wide variety of applications, but their uptake into current technologies has stalled due to the difficulty of their production and manipulation post-assembly. Therefore, new routes to readily control the assembly of nanoparticles represent an important area of research. I utilise the unique macrocyclic host-guest chemistry of CB[n]s in conjunction with gold nanoparticles to demonstrate a novel approaches to nanoparticle self-assembly. The aim of this action was to produce new gold nanoparticle structures to be used as constructs for Surface-Enhanced Raman Scattering (SERS), which is a sensitive light detection technique for molecules, to obtain (1) fundamental insights into SERS (2) perform and study (catalysed) chemical reactions, and (3) perform advanced molecular sensing, meaning detecting molecules at very low concentrations or detecting properties of molecules that were previously not accessible. Achieving these objectives opens up new avenues for the applications mentioned. Overall, the objectives of this action were nearly all achieved and most surpassed, with some objectives reached via unforeseen pathways.

Work performed

One goal was to perform SERS measurements on gold-silica nanostructures, to reach new understanding of SERS and perform ultrasensitive sensing. I developed a gold-silica construct which yields up to a hundredfold increase in SERS intensity compared to existing nano-architectures. The extreme field enhancements created meant that molecular properties which were previously inaccessible could now be probed.

A second goal was to achieve accurate chemical sensing in a bulk sample (i.e. not a nano-architecture on a surface as above). Working towards this goal, I discovered that adding a few microliters of methanol (MeOH) to a AuNP dispersion aggregating in the presence of CB[5] stops this aggregation. This prompted me to investigate the behaviour of the AuNP-CB[5] system in the presence of low concentrations of methanol. SERS measurements revealed the appearance of unexpected peaks in the spectra, increasing with methanol concentration. We used mathematical techniques to accurately identify and quantify the individual analyte components and correlate SERS intensities with analyte concentrations. Calculations suggested that molecules in or near the CB[5] cavity forming hydrogen bonds are responsible for the peaks observed. Moreover, using CB[5] and CB[6]-based self-assembly of gold nanoparticles into sensing aggregates, we demonstrated SERS-based detection of as little as 0.1 v/v% of methanol in water, and 4 v/v% methanol in ethanol/water mixtures (bordering upper consumption limits) [1].

A final goal was to study catalytic processes using SERS sensing. CB[8] host molecules containing methyl viologen (MV) in the cavity were sandwiched between a nanoparticle and a mirror. With a laser it is possible to photocatalytically induce different redox states of the MV molecule (from MV0 to MV1+ and MV2+) as evidenced by changing peak positions in the SERS spectra. By careful analysis of the peaks (via a tri-analyte analysis) we showed that the redox events are predominantly single-molecule events even though there are several hundred molecules in the nanogap. Moreover, the majority of MV molecules in the gap maintain their 2+â€‰state and contribute weakly to the SERS spectrum, while occasional MV molecules at the apex of the plasmonic hotspot undergo these redox processes and dominate the final spectra. This constitutes important advancements on redox reactions at the single molecule level [2] which can now be used to now study catalytic reactions in the above bulk samples as well.

[1] â€œSmart supramolecular sensing with cucurbit[n]urils: probing hydrogen bonding with SERSâ€, Faraday Discussions 205, 505-515 (2017).
[2] â€œPlasmonic tunnel junctions for single-molecule redox chemistryâ€, Nature Communications 8, 994 (2017).

Final results

In the first work package, a nanoplasmonic construct was developed that yields up to a hundred-fold increase in SERS intensity compared to existing constructs. This work has set new bench-mark in nanoparticle assembly as well as SERS sensing. Its impact on SERS studies performed in our lab will be large since the higher signals will expectedly reveal many other fundamental properties of molecules and reactions that were previously not distinguishable. In terms of socio-economic impact, such findings can change the way catalysis (chemical fabrication) and analysis (drug detection, disease diagnosis) are performed. Lasers powers used can also be decreased which is obviously of importance to the environment.

In the second work package, we demonstrated SERS-based detection of hydrogen-bonded species using gold-CB[5] constructs. This provides a powerful test bed for probing molecular dynamics at the nanoscale. We also showed detection of as little as 0.1% of methanol in water and 4% methanol in ethanol/water mixtures (bordering on upper consumption limits); this provides an interesting application for the purpose of food safety.

In the third work package, we developed a novel recoverable SERS substrate for measurements in the bulk. These reached detection limits down to the few micromolar range, meaning that the method is of interest for clinical use and for drug detection. We also studied redox reactions in a nanogap with surface based nano-architectures. The data gathered will be hugely helpful in further studies on photocatalytic reactions in plasmonic constructs and bears potential to photocatalytically improve important reactions (such as water splitting) on the macroscale.