I have run single particle samples as low as 1-10 ug/ml, although some literature has a 70 ug/ml suggested concentration. The standard negative stain recommendations are in the 100-300 ug/ml range. I would try your concentration on glow discharged grids and make sure you blot the sample onto the grid after absorption. Also run a sample free buffer as a control. If you sample contains phosphate, use filtered 1% phosphotungstic acid, pH 7.2 as a negative stain. Good luck