Graphene-DNAzyme Junctions: A Platform for Direct Metal Ion Detection with Ultrahigh Sensitivity

Abstract

Many metal ions are present in biology and in the human body in trace amounts. Despite numerous efforts, metal sensors with ultrahigh sensitivity (< a few picomolar) are rarely achieved. Here, we describe a platform method that integrates a Cu²⁺-dependent DNAzyme into graphene-molecule junctions and its application for direct detection of paramagnetic Cu²⁺ with femtomolar sensitivity and high selectivity. Since DNAzymes specific for other metal ions can be obtained through in vitro selection, the method demonstrated here can be applied to the detection of a broad range of other metal ions.

Fig. 1. (a) Schematic representation of graphene–DNAzyme junctions. (b) The structure of the Cu²⁺-sensitive DNAzyme and corresponding catalytic activity. The DNA substrate has been functionalized by amines on both ends for molecular connection (see the ESI†). The cleavage site is indicated by an arrow.

Fig. 2. (a) I–V curves of a graphene device before (black) and after (red) cutting. (b) Device characteristics of a representative DNAzyme-reconnected device after DNA connection (black) and further Cu²⁺ treatments (0.5 nM, 5 min) in the presence of 50 μM ascorbate (red). (c) I–V curves of a graphene device reconnected by control DNA, after DNA connection (black) and after further Cu²⁺ treatment for 5 min (red) (0.5 nM with 50 μM ascorbate).

Fig. 3. (a–d) Device characteristics of different graphene–DNAzyme junctions after DNA connection (black) and further Cu²⁺ treatments of different concentrations in the presence of 50 μM ascorbate (red) (a: 0.5 pM; b: 50 fM; c: 5 fM; d: 0.5 fM). (e) Statistical reaction times for the complete Cu²⁺ catalytic reactions at different concentrations. (f) Concentration-dependent dynamics of the Cu²⁺ catalytic reactions. All the measurements were performed at V _D = –50 mV.

Fig. 4. (a) I–V curves of DNAzyme-bridged graphene devices after DNAzyme connection (black) and after further treatments with Pb²⁺ (∼0.5 nM) under the same conditions in the presence of 50 μM ascorbate (red). (b) Statistical comparisons of conductance changes under the same conditions in the presence of 50 μM ascorbate (0.5 nM of Pb²⁺, Mg²⁺, Fe²⁺, Ni²⁺, Fe³⁺, Zn²⁺, Ca²⁺ and Cu²⁺; 5 mM of K⁺; 135 mM of Na⁺; 60 nM of Al³⁺). All the measurements were performed at V _D = –50 mV.