A bifunctional immunosensor based on osmium nano-hydrangeas as a catalytic chromogenic and tinctorial signal output for folic acid detection† Junkang Pan,a Qiyi He,ab Zhiting Lao,a Yikui Zou,a Jingyi Su,a Qinglan Li,a Zekai Chen,a Xiping Cui,*a Yanfei Cai*c and Suqing Zhao *a Author affiliations * Corresponding authors a Department of Pharmaceutical Engineering, School of Biomedical and Pharmaceutical Sciences, Guangdong University of Technology, Guangzhou 510006, People's Republic of China...