Abstract: A nano viscometer device suitable for determining the concentration of a solute within a fluid sample preferably includes a hollow core Photonic Crystal Fibre (HC-PCF) acting as a capillary tube having a core and means for filling the capillary tube with a fluid sample. Light is preferably guided light into the HC-PCF and detected exiting the tube. The rate at which the capillary tube is filled with the fluid is optically measured based on the light to determine the viscosity of the fluid to calculate the concentration of a solute. The preferred capillary viscometer is capable of measuring the viscosity of nano-litre quantities of a sample fluid. On one example, the preferred viscometer makes use of HC-PCF for the detection of glucose dissolved in nano water, demonstrating that HC-PCF can be used for continuous monitoring of glucose levels within blood plasma.