Birchall, Liesbeth
(2022)
Reactive inkjet of quantum dot-silicone composites.
PhD thesis, University of Nottingham.
Abstract
There is a need for high-resolution and high-sensitivity temperature sensing in fields such as micro/nanoelectronics, integrated photonics, and biomedicine; however, non-invasive integrated sensing is difficult and expensive to achieve in miniaturised devices, as fabrication is greatly complicated by multi-step processes, heat treatments, and material compatibility. Inkjet printing (IJP) is a direct writing technique in the material jetting AM category that is effective for maskless multi-material printing with <50 µm resolution, which enables production of end-use devices and could simplify sensor integration. Existing inkjet-printed temperature sensors comprise simple circuit devices, which use the change in the electrical resistance of a sensing area to measure temperature. While current examples are well-suited to wearable sensors, they do not achieve the spatial and thermal resolutions desired for printed devices such as microfluidics.
Development of inks for luminescence nanothermometry would enable inkjet-printable sensing geometries for planar and 3D thermal imaging with submicron and subdegree resolutions. Silicones are polymers suitable for optical sensing due to their ultraviolet (UV) and thermal stability, optical transparency, and high refractive indices. Composite inks for luminescence nanothermometry can be formulated with quantum dots (QDs), fluorescent semiconductor nanocrystals with intrinsic, reversible temperature quenching. Printable optical sensing materials would enable in situ temperature monitoring for applications and geometries that are otherwise impossible to monitor by conventional means.
This thesis describes the development of the first inkjet-printable QD-silicone composite, and the first ink for luminescence thermometry, for integrated optical sensing; these may also have use in lighting applications . 2-part addition cure silicone inks and 1-part UV cure silicone inks were explored and QD-silicone composites were synthesised; inkjet printing of an addition cure QD-composite was demonstrated.
Printing of reactive addition cure inks, where Ink A contained crosslinker and Ink B contained catalyst, was demonstrated using drop-on-drop IJP with the smallest average drop diameters reported for silicone IJP to date (33 36 µm). To overcome poor contact pinning, a pinned grid strategy was used for single printhead IJP and a line-by-line strategy for dual printhead IJP. Curing was the greatest challenge in reactive inkjet of QD-silicone composites, as labile ligands on the QDs poisoned the platinum catalyst despite low QD loading (0.005 wt% QD-Ink A). PtCl2 catalyst was added at low loading to enable curing and to explore the interactions between QDs and the catalyst. However, quenching was observed, with 70% decrease in emission intensity as PtCl2 concentration doubled; it was theorised that the QDs and catalyst competed for ligands, leading to metal-induced aggregation. Printing of fluorescent QD-silicone composites was demonstrated on a single printhead system using a pinned grid strategy; inks with no PtCl2 had stronger fluorescence but did not cure, highlighting their greater vulnerability to delays or fluctuations in heating.
Novel UV curable silicone inks were formulated for inkjet using a high throughput screening method. Two photoinitiators (PIs) were trialled: DMPA (2,2-dimethoxy-2-phenylacetophenone) and TPO (phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide). DMPA was associated with rapid loss of fluorescence in QD-silicones, whereas quenching was not observed with TPO. Detachment of passivating ligands followed by photo-oxidation was suggested as a mechanism: TPO radicals are more susceptible to recombination with oxygen radicals than DMPA derived radicals, which might result in better shielding of the QD surface. Printing of 1 wt% TPO silicone inks without quantum dots was carried out under nitrogen to prevent oxygen inhibition. Jetting was demonstrated with 34-42 µm average drop diameter on silanised glass slides, while printing of continuous films was demonstrated on glass slides coated in a release agent.
The temperature sensing performance of novel QD-silicone composites was assessed via fluorescence spectroscopy and imaging. 100 nm diameter QD clusters were observed in transmission electron microscopy and micron-scale QD aggregates in optical microscopy. QD emission appeared to be largely unchanged by immobilisation in silicone, although QD aggregation was expected to reduce photostability of the composite. Intensity- and spectral shift-based optical thermometry was demonstrated using well-plate reading and confocal laser scanning microscopy. Emission sensitivity at 627 nm was found to be approximately -0.7 to -1.2 % °C-1 between 30 50 °C and spectral sensitivity 0.07 to 0.08 nm °C-1, in agreement with other values in QD-sensing literature. Intensity decreased between thermal cycles of the same sample, although values at 60 °C were unchanged, while spectral shift appeared repeatable without redshift. Overall, fluorescent QD-silicone composites were produced via IJP for the first time and were shown to have temperature-sensitive emission. These materials are suitable for inkjet-printable devices with embedded optical temperature sensors using luminescence nanothermometry.
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