Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

group P2,,,; a =14.828(2). h = 11.513(1), c =16.637(2)A. 0 =112.735(4). V = 2619.5(5) A', 2 = 4 , pidlid =1.373 gem-,; p = 4.25 m m -' ; max. absorption correction factor l.60:20max =115,24';Cu,,radiation; T = 298K:O-28scan;reflectionscollectedjunique!used, 3767/3579 (R,,, = 0.0219)/3577; parameters refined, 352; Rl!xR2 [2794 reflections with I > 2a/], 0.0576/0.1579; GOF = 1.029; [ A / GI,,, = 0.003; [A&JAp]*," 0.373/ -0.308 eA-3, structure solution and refinement with the programs SHELXS-86 and SHELXL-93: H atoms of 1.10phenanthroline were located by difference maps and refined isotropically, the rest were introduced at calculated positions as riding on bonded atoms; all non-H atoms refined anisotropically (except atoms of the solvent methanol which refined isotropically with occupancy fixed at 10.5. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-179-98. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road.

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The solid state structure of the fluorinated If2,3,. is reported; it is the first dithiadiazolyl radical t o retain its paramagnetic character in the solid state.

Nineteen diiodophosphoranes, R,PI, (R, = Ph,, substituted triaryl, mixed arylalkyl, or trialkyl), the vast majority of which have previously not been reported, are described. Their "PP(H} NMR data indicate an ionic [R,PI]I structure in CDCI, solution, in keeping with results of other solution studies of a limited number of dihalogenophosphoranes, but solid-state ,lP-{H} magic angle spinning NM R data for R,PI, (R, = Ph,, PhMe, or Me,) indicate a molecular four-co-ordinate R,P-1-1 structure, strongly suggesting that this is the common structure for diiodophosphoranes, and not the five-co-ordinate trigonalbipyramidal structure of conventional wisdom.