Early and sensitive detection of avian influenza virus (AIV) such as H9N2 is needed as AIV threatens human health and the food industry. Here, we developed an ultrasensitive and naked-eye counting strategy for quantifying AIV by two probes. One probe was a complex of M13 phage and gold nanoparticles (GNPs) modified with antiphage and anti-AIV antibody. The other probe was magnetic nanoparticles modified with the same anti-AIV antibody. The two probes cocaptured target AIV to form a sandwich

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... cture. Because each M13-GNP probe contains multiple phages and each phage could be plated into an eye-visible plaque, the phages acted as a signal amplifier. After the phages from the sandwich structure were plated, the copy numbers of resultant plaques and target AIV were numerically correlated. Consequently, counting the number of the plaques with the naked eyes resulted in the quantification of the target AIV. The sensitivity of the naked-eye counting strategy for quantification of AIV was determined to be down to 50 PFU/mL, close to the sensitivity of digital polymerase chain reaction. With the feature of ultrasensitivity, the phage-GNP probeemediated naked-eye counting method holds a great promise for early visualized quantification of pathogenic viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).