All-fiber quantum relaxometry for biochemical sensing based on diamond NV centers

The longitudinal relaxation time (termed as T₁) of nitrogen-vacancy (NV) centers in nanodiamonds can be affected by surface electric or magnetic noise, which has been exploited to develop cutting-edge quantum relaxometry for biochemical sensing. In this work, a tiny all-fiber quantum probe based on longitudinal relaxometry was developed by chemically-anchoring nanodiamonds on the surface of a cone fiber tip. The dependences of T₁ on surface electric and magnetic noise were discussed in theories first and then experimentally demonstrated in varied pH and Gd³⁺ concentration solutions, respectively. Because of NV centers being subject to enhanced coupling from surface noise, T₁ reduced from 290 to 245 µs when pH changed from 3 to 9 and reduced to 220 µs when Gd³⁺ concentration increased to 10 mM, agreeing well with theoretical results. Based on these, the Gd³⁺-tagged-biotin and streptavidin model was designed and implemented on the all-fiber probe, and results demonstrated the detection of biotin with a limit of 168 nM and good specificity. This paper opens a new way to develop an all-fiber quantum probe by exploiting the unique electrical spin properties of NV centers, and the probe shows great potential for biological detection with high sensitivity and specificity.