A method of testing for the presence of a target nucleic acid sequence, target protein or target antigen comprising the steps of: exposing nucleic acid sequences, proteins or antigens from a sample to a probe under hybridisation conditions, the probe comprising a first catalytic element or precursor thereof and a binding element capable of binding at least partially to the target nucleic acid sequence, target protein or target antigen; exposing the nucleic acid sequences, proteins or antigens to: elements identifiable by photonic, optical or electrical measurement; a second catalytic element; at least a first and second substrate, said first substrate being capable of reaction with the first catalytic element and said second substrate being capable of reaction with the second catalytic element; and a conversion substrate; under conditions which result in a physical change of the conversion substrate and the physical change causes the elements to be entrapped or otherwise retained against removal by washing, substantially in the vicinity of the probe, wherein the physical change of the conversion substrate is effected by the product of the reaction of the first substrate and first catalytic element and the product of the reaction of the second substrate and second catalytic element, wherein the first catalytic element is a catalytic element that is capable of a turnover rate of greater than 3000 molecules per second. exposing nucleic acid sequences, proteins or antigens from a sample to a probe under hybridisation conditions, the probe comprising a first catalytic element or precursor thereof and a binding element capable of binding at least partially to the target nucleic acid sequence, target protein or target antigen; the probe comprising a first catalytic element or precursor thereof and a binding element capable of binding at least partially to the target nucleic acid sequence, target protein or target antigen; exposing the nucleic acid sequences, proteins or antigens to: elements identifiable by photonic, optical or electrical measurement; a second catalytic element; at least a first and second substrate, said first substrate being capable of reaction with the first catalytic element and said second substrate being capable of reaction with the second catalytic element; and a conversion substrate; elements identifiable by photonic, optical or electrical measurement; a second catalytic element; at least a first and second substrate, said first substrate being capable of reaction with the first catalytic element and said second substrate being capable of reaction with the second catalytic element; and a conversion substrate; under conditions which result in a physical change of the conversion substrate and the physical change causes the elements to be entrapped or otherwise retained against removal by washing, substantially in the vicinity of the probe, wherein the physical change of the conversion substrate is effected by the product of the reaction of the first substrate and first catalytic element and the product of the reaction of the second substrate and second catalytic element, wherein the first catalytic element is a catalytic element that is capable of a turnover rate of greater than 3000 molecules per second.