Direct
photoluminescence (PL) from metal nanoparticles (NPs) without
chemical dyes is promising for sensing and imaging applications since
this offers a highly tunable platform for controlling and enhancing
the signals in various conditions and does not suffer from photobleaching
or photoblinking. It is, however, difficult to synthesize metal NPs
with a high quantum yield (QY), particularly in the near-infrared
(NIR) region where deep penetration and reduced light scattering are
advantageous for bioimaging. Herein, we designed and synthesized Au–Ag
long-body nanosnowman structures (LNSs), facilitated by polysorbate
20 (Tween 20). The DNA-engineered conductive junction between the
head and body parts results in a charge transfer plasmon (CTP) mode
in the NIR region. The junction morphology can be controlled by the
DNA sequence on the Au core, and polythymine and polyadenine induced
thick and thin junctions, respectively. We found that the LNSs with
a thicker conductive junction generates the stronger CTP peak and
PL signal than the LNSs with a thinner junction. The Au–Ag
LNSs showed much higher intensities in both PL and QY than widely
studied Au nanorods with similar localized surface plasmon resonance
wavelengths, and notably, the LNSs displayed high photostability and
robust, sustainable PL signals under continuous laser exposure for
>15 h. Moreover, the PL emission from Au–Ag LNSs could be
imaged
in a deeper scattering medium than fluorescent silica NPs. Finally,
highly robust PL-based cell images can be obtained using Au–Ag
LNSs without significant signal change while repetitively imaging
cells. The results offer the insights in plasmonic NIR probe design,
and show that chemical dye-free LNSs can be a very promising candidate
with a high QY and a robust, reliable NIR PL signal for NIR sensing
and imaging applications.