Luminescent iridium(iii) complexes as solution-based and surface-active probes for optical detection of analytes

Carrod, Andrew James (2021). Luminescent iridium(iii) complexes as solution-based and surface-active probes for optical detection of analytes. University of Birmingham. Ph.D.

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Abstract

The development of luminescent Ir(III) complexes for application in small analyte sensing is of great interest in the scientific community. Iridium cyclometalated complexes provide ideal sensing probes with photo- and electro- chemical characteristics which are sensitive to changes in the local environment.
Consequently, we have designed and synthesised a series of cyclometalated iridium complexes based upon the archetypal [Ir(ppy)_2bpy]^+ compound (ppy = 2-phenylpyridine, bpy = 2,2’-bipyridine). This complex is emissive, at /lambda_max = 602 nm with a quantum yield of 14 % in deaerated acetonitrile. Previous studies have determined that bpy is the chromophoric ligand, and the lowest energy triplet states are mixed metal-ligand charge transfer (MLCT) and ligand-ligand charge transfer (LLCT) states.
We synthetically modify 2-phenylpyridine and 2,2’-bipyridine ligands to create analogues with desired functionality. Through synthesis of novel Ir(III) complexes based on these ppy and bpy analogues we incorporate desired functionality into a luminescent metal complex.
In our first approach a boronic acid moiety was appended to the 2-phenylpyridine ligands of the metal complexes, in either the 3- or 4- position of the phenyl ring. Boronic acid groups are widely reported as fluorescent chemosensors for saccharides; of which glucose is the most relevant to human physiology. Glucose concentration in the blood of patients is a good measure of the successful control and treatment of diabetes. Photophysical properties of the two novel iridium complexes Ir-4-BOH and Ir-3-BOH were investigated in deaerated acetonitrile to compared with the [Ir(ppy)2bpy]+ complex. We found both complexes were emissive with /lambda_max = 610 nm and 594 nm, for Ir-4-BOH and Ir-3-BOH respectively, with quantum yields of 12 and 8 %. We subsequently investigate the saccharide binding ability of the synthesised Ir(III) complexes in aqueous solutions at physiological pH. We utilise the dual photoluminescence and electrochemiluminescence response permitted by the use of the transition metal core to report on the binding event, and to allow a readout of the saccharide concentration. A 70 % maximum decrease in photoluminescence intensity and 90 % maximum decrease in electrochemiluminescence intensity for the Ir-4-BOH upon addition of fructose demonstrates that the emission intensity of the complexes is highly susceptible to the binding of saccharide. Formation of the boronic ester is probed by 11B and 19F nuclear magnetic resonance spectroscopy, and mass spectrometry. We have equally incorporated the Ir-4-BOH into hydrogel devices, which are shown to be non-toxic, stable, and have potential as wearable devices for the monitoring of saccharide concentration in interstitial fluid. This is the first report of the use of a transition metal complex with dual photoluminescence and electrochemiluminescent response to saccharide binding, and the first report of a functional saccharide sensing device based on a transition metal complex.
To extend the approach of analyte detection with Ir(III) complexes to functional devices, we report the formation of iridium transition metal probes containing a previously reported 2,2’-bipyridine ligand (bpySS) with a lipoic acid moiety for specific attachment to gold surfaces. The synthesised complexes IrC6bpySS and IrC12bpySS are also modified with hexyl or dodecyl hydrophobic chains respectively, this chain is incorporated on each of the 2-phenylpyridine ligands at the 2- position of the phenyl ring (C6 = 2-(2-hexylphenyl)pyridine and C12 = 2-(2-hexylphenyl)pyridine). The photophysical properties of the metal complexes are investigated in degassed acetonitrile where both have the same emission maxima, max = 606 nm, and show relatively small quantum yields of 3 % and 5 % for IrC6bpySS and IrC12bpySS respectively. Luminescent lifetimes of 210 and 260 ns are measured for IrC6bpySS and IrC12bpySS under the same conditions. The C6 containing compound has been previously synthesised and attached to gold nanoparticles for cell imaging and we expand on the use of hydrophobic metal complexes through attachment to planar and patterned gold surfaces. Select physical and chemical properties of the transition metal decorated gold surfaces are reported; with the surface morphology and chemistry probed through atomic force microscopy and X-ray photoelectron spectroscopy. Contact angle is used to probe the hydrophobicity of the surfaces, and sequentially increased hydrophobicity of the decorated gold surfaces is observed with increasing alkyl chain length. We propose that the hydrophobic nature of the iridium probes inferred by the alkyl chain on the cyclometalating ligand lends itself to the sensing of hydrophobic molecules, one analyte of interest is the persistent organic pollutant perfluorooctanoic acid. This pollutant is an environmental contaminant widely monitored by liquid chromatography-mass spectrometry, due to its known hepatotoxic and carcinogenic properties. The design of the iridium complex incorporates gold binding ligands with sufficient height to minimise quenching of the luminescence by the gold surface meaning luminescence studies may be undertaken using the photoluminescence signal of the Ir(III) complex. We assess the ability of the devices to sense fluorosurfactant concentrations through monitoring of the luminescence lifetime of the Ir(III) complex and observe augmentation of the lifetime at high fluorosurfactant concentrations. To improve the efficiency of sensing we detail attachment of the Zonyl FSA fluorosurfactant, and characterise these surfaces by time of flight secondary ion mass spectrometry. It is reasoned fluorine-fluorine interactions between Zonyl and the analyte will aid in the adsorption of the analyte. We observe high sensitivity and can report significant changes in the luminescence lifetime (710 to 860 ns) upon additions of pM concentrations of analyte rivalling reported electrochemical techniques. The sample preparation time is greatly reduced from the gold standard HPLC methods currently employed. This is the first report of a transition metal complex-based device for optical sensing of the environmental pollutant perfluorooctanoic acid and will provide a solid foundation for further optimisation of the binding event.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):