The transient currents induced by an applied direct voltage (DC) electric field have been measured in the nematic phase of liquid-crystal cells. The experimental results show that the addition of insulating nano-particles such as diamond powders, leads to a decrease in the ion concentration in cells so as to reduce transient currents and threshold voltage drastically. Simultaneously, a high voltage-holding ratio (VHR) is observed in doped cells. Such a high VHR, in comparison with the one in nematic liquid crystals doped by carbon nanotubes, is a superior feature for future liquid crystal display applications.