We present a strategy for the synthesis of water-soluble, monodisperse, highly fluorescent gold nanoclusters (Au NCs) with sizes of 1.8 nm by co-reduction of glutathione and citrate (denoted as GS/C–Au NCs) for 24 h at 50 °C. The high content of Au(I)–thiolate complexes (about 75%) on the surfaces of core–shell structured GS/C–Au NCs is responsible for their strong fluorescence generated by aggregation-induced emission (AIE). The advantages of this method using citrate are as follows: (i) lower reaction temperature; (ii) controlled formation rate of Au(0) cores by selective reduction of Au(III) ions; and (iii) enhanced colloidal stability of GS/C–Au NCs in the wide pH range from 4.1 to 8.6 due to differently stable states of glutathione–citrate complex on the surfaces. In addition, the fluorescence intensity of GS/C–Au NCs obtained is pH-dependent and can be reversibly adjusted in the pH range from 4.1 to 8.6 due to changes in their surface charge density stemming from the transitions among differently stable states of the glutathione–citrate complex. Our preliminary study also demonstrates that the resulting GS/C–Au NCs can be used as fluorescent nanoprobes in bio-imaging.