A method comprising: providing a probe comprising a target molecule-binding region linked to a structured nucleic acid region, wherein the structured nucleic acid region sequesters an HCR initiator; providing an unstructured trigger oligo free in solution that is not covalently linked to a domain that directly binds the target molecule, wherein the trigger oligo directly binds to the structured nucleic acid region, which changes conformation to expose the initiator; providing an HCR amplifier comprising a pair of directly labeled metastable HCR hairpin monomers; and amplifying from the exposed HCR initiator using HCR by self-assembling of the metastable HCR hairpin monomers to form an HCR amplification polymer; wherein when the trigger oligo binds to an input domain, it exposes an output domain which is capable of triggering HCR, wherein the structured nucleic acid region comprises a hairpin probe, and wherein the hairpin probe comprises: the input domain comprising a toehold and stem section, and the output domain comprising a hairpin loop and a complement to the stem section, such that the output domain comprises a sequestered HCR initiator; wherein the target molecule-binding region is an antibody, and wherein the antibody comprises a first antibody that binds to the target molecule, and a second antibody that directly binds to the first antibody, wherein the structured nucleic acid region is linked to the second antibody but is not linked to the first antibody, and wherein the second antibody does not directly bind the target molecule, wherein the metastable HCR hairpin monomers are not covalently linked to a domain that directly binds the target molecule. providing a probe comprising a target molecule-binding region linked to a structured nucleic acid region, wherein the structured nucleic acid region sequesters an HCR initiator; providing an unstructured trigger oligo free in solution that is not covalently linked to a domain that directly binds the target molecule, wherein the trigger oligo directly binds to the structured nucleic acid region, which changes conformation to expose the initiator; providing an HCR amplifier comprising a pair of directly labeled metastable HCR hairpin monomers; and amplifying from the exposed HCR initiator using HCR by self-assembling of the metastable HCR hairpin monomers to form an HCR amplification polymer; wherein when the trigger oligo binds to an input domain, it exposes an output domain which is capable of triggering HCR, wherein the structured nucleic acid region comprises a hairpin probe, and wherein the hairpin probe comprises: the input domain comprising a toehold and stem section, and the output domain comprising a hairpin loop and a complement to the stem section, such that the output domain comprises a sequestered HCR initiator; wherein the target molecule-binding region is an antibody, and wherein the antibody comprises a first antibody that binds to the target molecule, and a second antibody that directly binds to the first antibody, wherein the structured nucleic acid region is linked to the second antibody but is not linked to the first antibody, and wherein the second antibody does not directly bind the target molecule, wherein the metastable HCR hairpin monomers are not covalently linked to a domain that directly binds the target molecule. the input domain comprising a toehold and stem section, and the output domain comprising a hairpin loop and a complement to the stem section, such that the output domain comprises a sequestered HCR initiator; wherein the target molecule-binding region is an antibody, and wherein the antibody comprises a first antibody that binds to the target molecule, and a second antibody that directly binds to the first antibody, wherein the structured nucleic acid region is linked to the second antibody but is not linked to the first antibody, and wherein the second antibody does not directly bind the target molecule, wherein the metastable HCR hairpin monomers are not covalently linked to a domain that directly binds the target molecule.