Colorimetric detection of glucose in rat brain using gold nanoparticles.

Direct, selective, and sensitive determination of the physiologically important species involved in brain function has drawn much attention because understanding the chemical nature in such physiological and pathological events affords a platform for, for example, neurotransmission and the diagnosis and therapy of diseases. Although a number of methods have been developed to accomplish such a purpose, the ever-increasing interest in the interface between chemistry and physiology presents a pressing need to establish theoretically simple, less technically demanding methods that are more readily adaptable to physiological conditions to probe the chemical systems involved in brain functions. Gold nanoparticle (AuNP)-based colorimetric assays have been applied in a variety of research fields owing to the efficient integration of the unique optical properties of gold nanoparticles and the excellent surface/interface recognition ability afforded from the rational design of the surface chemistry of AuNPs. For instance, rational design of the surface chemistry of AuNPs promotes specific interactions between the receptors and analytes and, as a result, makes the measurements highly selective. Furthermore, the high extinction coefficient of AuNPs, particularly in relation to those of common organic chromophores, enables the colorimetric assay to be very sensitive. Moreover, colorimetric assays have advantages in their simplicity, both in the instrumentation used and in their operation. For example, detection is possible with the naked eye or can be concisely performed with UV/Vis spectrometry, without the requirements for either complicated instrumentation or much knowledge of the electron or energy transfer involved in the electrochemical or fluorescent systems. Whilst these advantageous properties make the colorimetric assays particularly attractive for the effective detection of physiologically important species in cerebral systems, such systems have not been reported so far. Herein, we describe a colorimetric assay for the detection of cerebral glucose. In cerebral systems, glucose not only represents the primary energy source for the brain, but also plays important roles in synaptic transmission. Whilst some methods, typically electrochemical, have been developed for the detection of glucose, the complexity of cerebral systems presents a great challenge to these existing methods to provide glucose detection in cerebral systems with theoretical and technical simplicity. The rationale for the colorimetric assay of cerebral glucose is essentially based on in vivo microdialysis, and the aggregation of AuNPs induced by glucose through cascade reactions involving glucose, H2O2, and COH (Scheme 1). To the best of our knowledge, this is the first example in which a AuNP-based colorimetric assay has been utilized to detect physiologically important species in a cerebral system. This capability is envisaged to be applicable to monitoring brain chemistry in a simple fashion. This assay utilizes the aggregation-induced changes in the color and UV/Vis spectrometry associated with 13 nm gold nanoparticles. Initially, AuNPs that were synthesized with