Human epidermal growth receptor 2 (Her2) is a gene which plays a major role in breast cancer development. The quantification of Her2 expression in single cells is limited by several drawbacks in existing fluorescence-based single molecule techniques, such as low signal-to-noise ratio (SNR), strong autofluorescence and background signals from biological components. For rigorous genomic quantification, a robust method of orthogonal detection is highly desirable and we demonstrated it by two non-fluorescent imaging techniques -transient absorption microscopy (TAM) and second harmonic generation (SHG). In TAM, gold nanoparticles (AuNPs) are chosen as an orthogonal probes for detection of single molecules which gives background-free quantifications of single mRNA transcript. In SHG, emission from barium titanium oxide (BTO) nanoprobes was demonstrated which allows stable signal beyond the autofluorescence window. Her2 mRNA was specifically labeled with nanoprobes which are conjugated with antibodies or oligonucleotides and quantified at single copy sensitivity in the cancer cells and tissues. Furthermore, a non-fluorescent super-resolution concept, named as second harmonic super-resolution microscopy (SHaSM), was proposed to quantify individual Her2 transcripts in cancer cells beyond the diffraction limit. These non-fluorescent imaging modalities will provide new dimensions in biomarker quantification at single molecule sensitivity in turbid biological samples, offering a strong cross-platform strategy for clinical monitoring at single cell resolution.
Fluorescence-based single molecule techniques to interrogate gene expression in tissues present a very low signal-to-noise ratio due to the strong autofluorescence and other background signals from tissue sections. This report presents a background-free method using second-harmonic generation (SHG) nanocrystals as probes to quantify the messenger RNA (mRNA) of human epidermal growth receptor 2 (Her2) at single molecule resolution in specific phenotypes at single-cell resolution directly in tissues. Coherent SHG emission from individual barium titanium oxide (BTO) nanoprobes was demonstrated, allowing for a stable signal beyond the autofluorescence window. Her2 surface marker and Her2 mRNA were specifically labeled with BTO probes, and Her2 mRNA was quantified at single copy sensitivity in Her2 expressing phenotypes directly in cancer tissues. Our approach provides the first proof of concept of a cross-platform strategy to probe tissues at single-cell resolution in situ.
Cell-specific information on quantity and localization of key mRNA transcripts in single-cell level are critical to the assessment of cancer risk, therapy efficacy, and effective prevention strategies. However, most available technologies for mRNA detection rely on cell extraction that inherently destroys the tissue context and provide only average expression levels from cell populations or whole tissues. In this paper, we proposed a novel super resolution concept, second harmonic generation (SHG) super-resolution microscopy (SHaSM), and apply that to detect single short mRNA transcript, Her2 mRNA, beyond the diffraction limit. Nano-sized SHG crystals, barium titanium oxide BaTiO3 (BTO), were functionalized with two complimentary strands of Her2 mRNA after the chemical surface-modification. Dimer schematic was used to improve the specificity of detection and quantification, where two BTO monomers bind to the Her2 mRNA to form a dimer and being visualized via the SHaSM. SHaSM is able to detect single BTO nanocrystal with ~20 nm spatial resolution, and differentiate BTO dimers (Her2 mRNA) from BTO monomers (non-specific bounded BTO nanocrystal) with high specificity.
Engineering of quantum emissions is regarded as the heart of nano-optics and photonics; local density of optical states (LDOS) around the quantum emitters are critical to engineer quantum emissions, thus detection of the LDOS will impact areas related to illumination, communication, energy, and even quantum-informatics. In this report, we demonstrated a far-field approach to detect and quantify the near-field LDOS of a nanorod via using CdTe quantum dots (QDs) tethered to the surface of nanorods as beacons for optical read-outs. The spontaneous decay of QD emission in the proximity of nanorod was used as a ruler for elucidating the LDOS. Our analysis indicates that the LDOS of the nanorod at its ends is 2.35 times greater than that at the waist. Our approach can be applied for further evaluation and elucidation of the optical states of other programmed nanostructures.