Apparatus for plasmon resonance-based sensing of a plurality of different biological targets, the apparatus comprising: a biosensor array for plasmon resonance-based sensing of a plurality of different biological targets simultaneously, the biosensor array comprising a transparent substrate having a surface bearing a plurality of assay spots for plasmon resonance sensing, each of said assay spots comprising a discrete metallic island, said metallic island comprising a plurality of metallic nanoparticles (202a, 202b) to which are attached functionalising molecules for binding to said biological target, different ones of said islands bearing different ones of said functionalising molecules for binding to different ones of said biological targets, and wherein total internal reflection of light at said surface at a wavelength at or near said plasmon resonance results in scattering of said light away from said surface, said scattering being modulated by said binding of said biological targets, wherein said nanoparticles (202a, 202b) comprise adjacent pairs of nanoparticles (202a, 202b), the nanoparticles (202a, 202b) of each pair separated by a gap (g) of less than 100nm, each nanoparticle (202a, 202b) of said adjacent pairs having a length which is approximately equal to an odd integral number of half-wavelengths of said plasmon resonance, such that each said pair forms an optical antenna for said light; wherein said transparent substrate is transparent for illumination light wavelengths at or near said plasmon resonance; means for coupling light of at least one wavelength into the biosensor array such that the total internal reflection of said light at said surface generates an evanescent wave field which excites plasmons in said functionalised metallic nanoparticles (202a, 202b) and scatters said light; means for flowing a fluid carrying a plurality of said biological targets for analysis over said biosensor array; means for imaging said scattered light from said biosensor array to generate image data for said biological targets carried by said fluid; and means for analysing said image data to determine levels of said biological targets carried by said fluid. a biosensor array for plasmon resonance-based sensing of a plurality of different biological targets simultaneously, the biosensor array comprising a transparent substrate having a surface bearing a plurality of assay spots for plasmon resonance sensing, each of said assay spots comprising a discrete metallic island, said metallic island comprising a plurality of metallic nanoparticles (202a, 202b) to which are attached functionalising molecules for binding to said biological target, different ones of said islands bearing different ones of said functionalising molecules for binding to different ones of said biological targets, and wherein total internal reflection of light at said surface at a wavelength at or near said plasmon resonance results in scattering of said light away from said surface, said scattering being modulated by said binding of said biological targets, wherein said nanoparticles (202a, 202b) comprise adjacent pairs of nanoparticles (202a, 202b), the nanoparticles (202a, 202b) of each pair separated by a gap (g) of less than 100nm, each nanoparticle (202a, 202b) of said adjacent pairs having a length which is approximately equal to an odd integral number of half-wavelengths of said plasmon resonance, such that each said pair forms an optical antenna for said light; wherein said transparent substrate is transparent for illumination light wavelengths at or near said plasmon resonance; means for coupling light of at least one wavelength into the biosensor array such that the total internal reflection of said light at said surface generates an evanescent wave field which excites plasmons in said functionalised metallic nanoparticles (202a, 202b) and scatters said light; means for flowing a fluid carrying a plurality of said biological targets for analysis over said biosensor array; means for imaging said scattered light from said biosensor array to generate image data for said biological targets carried by said fluid; and means for analysing said image data to determine levels of said biological targets carried by said fluid.