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Page 2 of 33 Mao et al. Chem Synth 2023;3:26 https://dx.doi.org/10.20517/cs.2022.41

INTRODUCTION
Colloidal semiconductors with tunable size, morphologies, and compositions ranging from hundreds to
thousands of atoms exhibit unique physical, optical, and electronic properties . Over the past few decades,
[1-3]
numerous colloidal semiconductors have been reported to address challenges related to environmental,
healthy, and energy challenges. However, their efficacy has gradually weakened . To date, various
[4,5]
strategies have been used to enrich the properties of colloidal semiconductors , including doping,
[6,7]
coupling, and modulation (epitaxial or nonepitaxial growth) methods for tunable bandgap engineering.
This has resulted in superior electronic and optical properties and led to diverse applications in areas such
as solar energy harvesting, imaging, display, and biological sensing.

Copper is an abundant metal with low toxicity, lower costs, and environmental compatibility, while copper
chalcogenide semiconductors present a wide variety of compositions, sizes, and crystal structures [8-10] . This
extremely versatile set of ingredients offers not only analogous properties to Cd- and Pb-based chalcogenide
colloidal semiconductors but also possesses unparalleled features compared to the conservative nanocrystals
(NCs), particularly in terms of photothermal and plasmonic properties [11-14] . Moreover, the synthesis
protocol for colloidal copper chalcogenide NCs is still largely underdeveloped and does not yet reach the
same level as traditional Pb- or Cd-based chalcogenides [15-19] , which are still worthy of merit attention [20-24] .

The copper atom has high reaction activity and a fast diffusion rate, making it easy to be oxidized and form
monovalent or divalent copper ions. This leads to the formation of abundant defects in the lattice and
different components with special properties, including the plasmonic feature observed in heavily doped
copper-based chalcogenides. Compared with the noble metals (with a high density of free electrons and
exhibiting strong light-matter interaction properties) [25-27] and n-type plasmonic semiconductors , heavily
[28]
doped copper-based chalcogenides (including exotic-doping and self-doping) [22,29-33] present p-type features
with a lower density of free charge carriers [34-36] , resulting in longer wavelength LSPR absorption. Based on
the above-described properties, plasmonic Cu-chalcogenide NCs have attracted significant attention due to
their various applications, including the hotspot research field, such as copper-based radioimmune therapy
and cuproptosis.

Inspired by the aforementioned potential applications, in this review, we summarize the recent advances in
the heavily doped colloidal Cu-chalcogenide from synthesis to biological applications. Firstly, we provide a
brief introduction of classifications for copper chalcogenide NCs and discuss the extensively reported
synthesis methods. Next, we present theoretical studies based on doped plasmonic Cu-chalcogenide NCs
with LSPR properties. We also review the doping strategies for the formation of doped Cu-chalcogenide
NCs. Subsequently, we discuss the wide range of applications for colloidal plasmonic Cu-chalcogenide NCs,
including biosensing, in vivo imaging, diagnosis, and cancer therapy. Finally, we provide insights into the
opportunities and challenges of these potential applications for the next generation.

CLASSIFICATION OF COPPER CHALCOGENIDE NCS AND SYNTHESIS STRATEGY
The copper chalcogenide compounds were initially discovered in minerals, which is why most of them are
named Chalcocite, Djurleite, Roxbyite, Digenite, Anilite, and Covellite. For bulk Cu S crystals, the band gap
2
is 1.2 eV [37-40] . However, the one-valence copper atom is readily oxidized by the oxygen in the air, which
leads to the formation of the Cu S (x = 0~1) compound along with a copper vacancy, resulting in the p-
2-x
type feature. In Figure 1, the number of copper vacancies is one of the main factors for the formation of
diverse Cu S crystal structures, including hexagonal, cubic, monoclinic, and orthorhombic crystal structures.
2-x
Simultaneously, the rearrangement of the S atom (usually very slow) during the crystallization results in the
formation of numerous metastable stages, such as chalcocite (Cu S), djurleite (Cu S), roxbyite (Cu S),
1.81
2
1.94