An upconversion nanoparticle with orthogonal emissions using dual nir excitations for controlled two-way photoswitching

Article abstract of DOI:10.1002/anie.201408219

Developing multicolor upconversion nanoparticles (UCNPs) with the capability of regulating their emission wavelengths in the UV to visible range in response to external stimuli can offer more dynamic platforms for applications in high-resolution bioimaging, multicolor barcoding, and driving multiple important photochemical reactions, such as photoswitching. Here, we have rationally designed single-crystal core-shell-structured UCNPs which are capable of orthogonal UV and visible emissions in response to two distinct NIR excitations at 808 and 980 nm. The orthogonal excitation- emission properties of such UCNPs, as well as their ability to utilize low-power excitation, which attenuates any local heating from the lasers, endows the UCNPs with great potential for applications in materials and biological settings. As a proof of concept, the use of this UCNP for the efficient regulation of the two-way photoswitching of spiropyran by using dual wavelengths of NIR irradiation has been demonstrated.

Full text of DOI:10.1002/anie.201408219

Angewandte
Chemie
DOI: 10.1002/anie.201408219
Core–Shell Nanoparticles
An Upconversion Nanoparticle with Orthogonal Emissions Using Dual
NIR Excitations for Controlled Two-Way Photoswitching**
Jinping Lai, Yixiao Zhang, Nicholas Pasquale, and Ki-Bum Lee*
Abstract: Developing multicolor upconversion nanoparticles
(UCNPs) with the capability of regulating their emission
wavelengths in the UV to visible range in response to external
stimuli can offer more dynamic platforms for applications in
high-resolution bioimaging, multicolor barcoding, and driving
multiple important photochemical reactions, such as photo-
switching. Here, we have rationally designed single-crystal
core–shell-structured UCNPs which are capable of orthogonal
UV and visible emissions in response to two distinct NIR
excitations at 808 and 980 nm. The orthogonal excitation–
emission properties of such UCNPs, as well as their ability to
utilize low-power excitation, which attenuates any local heating
from the lasers, endows the UCNPs with great potential for
applications in materials and biological settings. As a proof of
concept, the use of this UCNP for the efficient regulation of the
two-way photoswitching of spiropyran by using dual wave-
lengths of NIR irradiation has been demonstrated.
developed UCNPs with tailored emission profiles, multicolor
UCNPs capable of regulating their emission wavelengths
from the UV to visible range in response to external stimuli
are garnering much interest recently, as they can offer more
dynamic platforms for applications in high-resolution bio-
imaging, multicolor encoding, and photoswitching.[6] How-
ever, there are only a few reports on such multicolor UCNPs.
A typical example is an excitation-responsive UCNP, whose
emission can be modulated between spectrally pure visible
light and mixed UV/Vis emissions by changing the power
density of 980 nm NIR excitation.[6b–d] This unique property of
the UNCPs was then successfully applied toward driving
important chemical reactions and their subsequent applica-
tions such as the two-way photoswitching of dithienyleth-
ene,[6b] the reversible control over the reflection of liquid
crystals,[6c] and modulating the biocatalytic activity of bac-
teria.[6d] However, the use of high-power 980 nm NIR light in
such UCNP systems, although advantageous, has been shown
to cause severe local heating, which has detrimental effects on
both solid-state devices and biological systems.[7] Moreover,
the spectrally mixed UV/Vis emission of these UCNPs
compromises the photoswitching systemꢀs ability to reliably
encode and transmit information in a spatiotemporally con-
trolled manner. Thus, to overcome these limitations, there is
a clear need to develop multicolor UCNPs capable of
selectively generating spectrally resolved emissions in the
UV and visible regions using external stimuli with negligible
heating effects.
L
anthanide-doped upconversion nanoparticles (UCNPs)
have recently gained much attention due to their unique
capability of upconverting low energy near-infrared (NIR)
light to high-energy ultraviolet (UV) and visible light.[1]
Combined with other excellent photophysical properties,
including long emission lifetimes, narrow emission band-
widths, and high photostability, UCNPs have shown wide-
spread application in fields ranging from bioimaging and
sensors to photovoltaics and solid-state devices.[2] One of the
critical requirements for harvesting their full potential is to
develop UNCPs with emission profiles specifically tuned
toward their target applications. This includes synthesizing
UCNPs with strong UVemission to trigger chemical reactions
such as the photocleavage of photolabile groups,[3] and highly
visible emitting UCNPs for nanomedicine-based applications
such as photodynamic therapy (PDT),[4] as well as NIR-
emitting UCNPs for in vivo bioimaging.[5] Among the various
We herein describe the design and synthesis of a
novel
core–shell
structured
b-NaYF4:Nd3+/Yb3+/
Tm3+@NaYF4:Nd3+@NaYF4@NaYF4:Yb3+/Er3+
UCNP
(Tm@Er) possessing dual NIR excitations (808 and 980 nm)
and the corresponding orthogonal emissions in the UV (347–
475 nm)/visible (545 nm) range by using low-power-density
excitation for minimal heating effects (Scheme 1). The unique
photophysical properties of these UCNPs represent a critical
advance in many applications involving the construction of
optical storage devices for information storage and trans-
mission, developing advanced drug delivery systems which
are capable of the sequential delivery of therapeutics, and
multicolor bioimaging and sensing. As a proof-of-concept
experiment, we demonstrate the highly efficient two way
photoswitching of spiropyran regulated by a single type of
UCNP with dual NIR excitations.
The general structure of the UCNP contains a core and
multiple shells, including a luminescent core (LC), an internal
photon-inert shell (IS), and an outer luminescent shell (LS;
Scheme 1a). In our developed UCNPs, the LC and LS contain
different sensitizers and activators to generate orthogonal
excitation and emission properties and the IS prevents energy
[*] J. Lai, Y. Zhang,[+] N. Pasquale,[+] K.-B. Lee
Department of Chemistry and Chemical Biology, Rutgers University
Piscataway, NJ 08854 (USA)
E-mail: kblee@rutgers.edu
Homepage: http://kblee.rutgers.edu/
[+] These authors contributed equally to this work.
[**] We acknowledge the kind support from Prof. Riman for fluorescent
lifetime measurements. K.-B. Lee acknowledges financial support
from the NIH Director’s Innovator Award [1DP20D006462-01], the
National Institute of Biomedical Imaging and Bioengineering of the
NIH [1R21NS085569-01], the N.J. Commission on Spinal Cord
grant [09-3085-SCR-E-0], and the Rutgers Faculty Research Grant
Program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408219.
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shell excitation–emission properties will be achieved, in which
the LS and LC will have individual emissions corresponding
to individual excitations at 980 nm and 808 nm, respectively.
Finally, Tm3+ and Er3+ were selected as activators of UCL due
to their spectroscopic characteristics in the UV and visible
region, respectively. In this work, two types of UCNPs were
synthesized, henceforth abbreviated as Tm@Er UCNPs and
Er@Tm UCNPs. Tm@Er UCNPs have Tm3+ doped in the LC
and Er3+ doped in the LS, displaying UV-blue emission from
the LC under 808 nm excitation and green emission from the
LS under 980 nm excitation. Er@Tm UCNPs have a reversed
core–shell composition with regard to activator doping and
therefore reversed emissions. This difference in their emissive
properties demonstrates the ability to orthogonally and
selectively tune individual emissions based on the choice of
activators of UCL (for more details see the main text
discussion).
We first synthesized the Tm@Er core–shell UCNPs
(Figure 1a). The UV-blue LC with 808 nm excitation was
synthesized according to reported methods.[8g,h] It is com-
Scheme 1. a) General structural design of the core–shell-structured
UCNPs. b) Illustration of the composition of the core–shell UCNPs
and the simplified energy level diagram for the photon upconversion
under NIR excitations. Yb3+ was selectively co-doped with and without
Nd3+ into the LC and LS as a sensitizer, to endow the LC and LS with
an excitation wavelength of 808 nm and 980 nm, respectively. Doping
of Nd3+ also leads to enhanced cross-relaxation between Nd3+ and
local activators, which quenches the UCL of the LC and thus endows
the LC with a selective excitation of 808 nm with high power density.
This structural design leads to orthogonal emissions in the lumines-
cent core–shell under dual NIR excitations. c) Direct and indirect two-
way photoswitching of spiropyran by using UV/visible light and using
Tm@Er UCNPs with dual NIR excitations.
migration between the two luminescent layers. Following this
general design, NaYF4 was chosen as the host matrix because
of its low lattice phonon energies and high upconversion
efficiency.[1d] We then doped the LS with Yb3+ as a sensitizer,
while co-doping the LC with both Nd3+ and Yb3+ (Sche-
me 1b). This specific arrangement of sensitizers affords an
excitation of 980 nm to the UCNP LS, while the LC is
responsive to both 980 nm and 808 nm excitations due to the
separate main absorption peaks of Yb3+ and Nd3+ which
locate at 980 and 808 nm, respectively.[8] It should be noted
that Nd3+ cannot transfer its excitation energy to activators
directly. As such, it requires the use of Yb3+ as a co-sensitizer,
acting as an energy bridge.[8f–h] However, the presence of Nd3+
in the LC not only imbues the LC with 808 nm excitation, but
also significantly decreases the upconversion luminescence
(UCL) of the LC due to the enhanced cross-relaxation
between Nd3+ and locally positioned activators (quenching
effect of Nd3+,[8h] for more details see the Supporting
Information (SI), Figure S1). As a result, the Nd3+-doped
LC becomes selectively excited under high-power 808 nm
excitation, with no emission observed from low-power 980 nm
excitation. Additionally, a layer of NaYF4, without any
dopant, was grown in between the LC and LS, as it can
serve as a photon inert matrix which prevents energy
migration between the LC and LS, which preserves their
individual and spectrally pure excitation–emission proper-
ties.[9] As such, UCNPs with individual and orthogonal core–
Figure 1. a) Composition of each layer of Tm@Er UCNPs. The false
color of each layer represents the corresponding nanoparticles shown
in (b–e). b–e) Low-resolution TEM images of Tm@Er UCNPs con-
structed by epitaxial layer by layer growth. f) High-resolution TEM
characterization of a single Tm@Er UCNP. The lattice extending
across the particle without interruption is indicative of its monocrystal-
line structure.
prised of a core (Figure 1b) doped with Yb3+, Nd3+, and Tm3+,
as well as a sensitized shell (Figure 1c) doped with Nd3+ in
high concentration. This core–shell structure minimizes the
cross-relaxation between Nd3+ and the local activators (Tm3+)
in the core due to the low Nd3+ dopant concentration
(1 mol%), but maximizes the absorption of 808 nm excitation
in the shell due to the high Nd3+ concentration (20%),
endowing the nanoparticles with 808 nm excitation at low
power density which minimizes local heating effects. As
explained previously, to block the energy migration between
the LC and LS, a layer of photon-inert NaYF4 was further
deposited through epitaxial growth (Figure 1d). Finally,
a green-colored LS, composed of NaYF4:Yb/Er was epitax-
ially deposited, leading to nanoparticles with a size of 41 ꢁ
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52 nm (width ꢁ length, Figure 1e). The shape evolution phe-
nomena during the shell growth of the nanoparticles were
ascribed to the kinetically favored anisotropic shell growth
during the coating process, which was also observed by other
groups.[8i,10] The high-resolution transmission electron micros-
copy (HRTEM) image shown in Figure 1 f shows the (100)
crystallographic planes of the Tm@Er UCNPs and demon-
strates that the as-synthesized multishell UCNPs were single
hexagonal-phase crystals. Additionally, the energy-dispersive
X-ray spectroscopy (EDX) spectrum indicates the presence
of the elements Na, F, Y, Yb, Nd, Er, and Tm, which confirms
the composition of the nanoparticles (see SI, Figure S2).
Next, we studied the upconversion profile of the as-
synthesized UCNPs under different wavelength laser excita-
tions. As shown in Figure 2a, the LC of the Tm@Er UCNPs
displays the characteristic emission peaks of Tm3+ in the UV-
tion of LC by 808 nm excitation. In contrast, the UCNPs
display strong characteristic emissions from Er3+ but rela-
tively weak emissions from Tm3+ under 980 nm excitation
with a power density of 60 WcmÀ2 (Figure 2b, bottom curve),
indicating that both the LC and LS are excited as both of them
use Yb3+ as sensitizer. It should be noted that the relatively
weaker emissions of Tm3+ in LC than that of Er3+ in LS may
be due to 1) the quenching effect of Nd3+ in LC,[8h] and 2) the
nonlinear excitation nature of UCNPs whereby the UV
emissions from Tm3+ show a stronger dependence on
excitation density than the visible emissions of Er3+ (a four
photon versus three photon process).[6b] Consequently,
decreasing the excitation power density eliminated the Tm3+
emissions more quickly than those of Er3+, and excitations of
40 WcmÀ2 or lower led to the selective emissions of Er3+ in
the LS only (Figure 2b, bottom curve). This result demon-
strates that the emission of Tm@Er under 980 nm excitation
can be easily modulated by regulating the excitation power/
wavelength, and that the LS can be selectively excited by
utilizing a low excitation power density due to the cooperative
effect of the Nd3+ doping, the photon-inert shell, and the
nonlinear excitation properties of UCNPs,[6b,c,11] as repre-
sented in Scheme 1b. Accordingly, a hexane solution of these
nanoparticles displays two different colored bands, blue and
green, under 808 nm and 980 nm excitation, respectively, with
excitation power densities of 40 WcmÀ2 or lower, demon-
strating the orthogonal excitation–emission property of the
Tm@Er UNCPs. Control experiments demonstrate that
UCNPs with the same core–shell structure, but without the
critical internal photon-inert shell, display strong energy
migration and do not possess separate core–shell excitation–
emission properties (Figure S3).
To further demonstrate the rationale behind the structure
of the proposed UCNPs, another type of UCNP, Er@Tm, was
synthesized based on the inverse composition of the activa-
tors Er3+ and Tm3+ (Figure S4). In contrast to Tm@Er
UNCPs, the Er@Tm UNCPs show incomplete separation of
the core–shell excitations and emissions. Accordingly, 980 nm
laser excitation stimulated not only UV-blue emissions from
Tm3+ in the LS, but also green emissions from Er3+ in the LC
(Figure S4 f), indicating a simultaneous excitation of the
luminescent core and shell by 980 nm irradiation at high
power density. The relatively weaker emission from Er3+ in
the LC is mainly due to the quenching effect of Nd3+ on
locally placed activators. Decreasing the excitation power
density can minimize the emission from Er3+, but it will also
result in a decrease in UV emission from Tm3+ (Figure S5).
These results strongly support the rational design of the
Tm@Er UCNPs, which have orthogonal core–shell excita-
tion–emission properties at distinct wavelengths to yield
spectrally pure UV-blue and green emissions. Furthermore,
since 808 nm and low power density 980 nm NIR light both
minimize any local heating from the lasers (Figure S6), these
UCNPs have unique advantages for applications in materials
and biological studies, e.g., for driving important photoreac-
tions with high spatiotemporal control and for reversibly
modulating the structure and properties of molecules such as
photoswitches, especially for advancing multicolor UCNP-
based photoswitching systems which require distinct excita-
Figure 2. a) Upconversion luminescence profiles of the luminescent
core with photon-inert shell and b) the Tm@Er UCNPs under different
wavelength excitations and varying power densities. The structures of
the corresponding nanoparticles are illustrated in the right panel. The
photographs show the UCNPs in hexane solution (1 wt%) under
irradiation by two laser beams at wavelengths of 808 nm and 980 nm.
The power density for 808 nm and 980 nm lasers is 60 WcmÀ2
.
blue region when excited using 808 and 980 nm lasers, due to
the presence of Nd3+ and Yb3+ in the luminescent core.
Accordingly, a hexane solution of these core nanoparticles
displays a deep blue colored band under 808 nm and 980 nm
laser irradiation, which is easily seen by the naked eye
(Figure 2a, right panel). For Tm@Er, only the characteristic
emission peaks of Tm3+ can be seen under 808 nm irradiation
(Figure 2b, upper curve), demonstrating the selective excita-
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tion–emission peaks, high photostability, and minimal photo-
inducible damage.
based two-way photoswitching ability of the synthesized
Tm@Er UCNPs (Figure 3a).
UCNP-based photoswitching systems, in which UCNPs
serve as an antenna and deliver high energy UV/Vis light
through the upconversion of low energy NIR light, are
capable of the spatiotemporal and reversible modulation of
the structure and properties of molecules.[6b,c,g,11] Such systems
can overcome the limitations of traditional UV/Vis light-
based photoswitching, which include photobleaching, toxicity,
and poor penetration through biological tissues. Spiropyran is
one of the most studied photoswitchable molecules known to
date and is widely used in the development of photoswitches,
molecular machines, and sensors.[12] Biomolecules such as
nucleic acids, enzymes, cellular receptors, and ion channels
have been functionalized with SP to remotely and orthogo-
nally regulate cellular behavior including gene transcription,
enzyme activity, and flux through ion channels with light.[13]
As illustrated in Scheme 1c, the photoisomerization between
spiropyran (SP) and merocyanine (MC) can be regulated by
UV and visible light.[12a] Importantly, the maximum absorp-
tion peaks of SP and MC that locate at 342 nm and 560 nm,
overlap well with the UV emission from the Tm@Er UCNPs
under 808 nm excitation and the green emission of our
Tm@Er UCNPs under 980 nm excitation, respectively (Fig-
ure 3b). As such, we sought to utilize this well-characterized
photoswitch in our system to demonstrate the NIR-light-
The photoswitching of SP by Tm@Er UCNPs was
performed with a simple mixture of SP and Tm@Er (for
details see Figure S7). As shown in Figure 3b, irradiation of
the colorless solution of SP and UCNPs with an 808 nm laser
leads to the photoisomerization of SP to MC, resulting in
a bright pink solution which is characterized by a red shift in
the UV peak as well as a dramatic enhancement of the
absorption centered at 560 nm. These results indicate that the
ring-opening reaction of the SP to MC was effectively driven
by the excitation at 808 nm. Based on the kinetics monitoring
of the photoswitching reaction using the characteristic
absorption of MC at 560 nm (Figure 3c), 92% of the photo-
stationary state can be obtained within 120 seconds with
808 nm excitation with a power density of 40 WcmÀ2. There-
after, the NIR-light-driven photoisomerization of MC to SP
was performed by irradiating the solution with a 980 nm laser.
The solution quickly turns back to colorless and the absorp-
tion spectrum shows a blue shift in the UV absorption band
and a decrease in intensity at 560 nm (Figure 3b). The kinetics
of this photoreaction under 980 nm irradiation with Tm@Er
UCNPs is much faster than that of the reaction in the dark
(Figure 3c). After 980 nm irradiation for 90 seconds with
a power density of 15 WcmÀ2 the absorption peak of MC no
longer exists, indicating a complete photoisomerization
reaction of SP to MC. These results demonstrate the
successful two-way photoswitching of spiropyran, which is
effectively driven using Tm@Er UNCPs with dual NIR
excitations at low power density.
We further tested the integrity of SP photoswitching by
using Tm@Er UCNPs with low power density 808 nm and
980 nm laser excitations, shown in Figure 3d. A schematic for
the “remote control” two-way photoswitching of SP and the
Tm@Er UNCPs by using two NIR lasers is shown in
Figure 3a. It is worth noting that there is more than 90%
retention of SP after five cycles of photoswitching, indicating
the robustness and reversibility of our UNCP based photo-
switching system, which is mainly due to the attenuation of
detrimental heating effects and photobleaching by using NIR
light with low excitation power density. Finally, we demon-
strated the construction of mesoporous silica-coated Tm@Er
(UCNP@MSN), which allow the UCNPs to be easily dis-
persed in aqueous solutions and undergo facile surface
chemistry for further functionalization and application (Fig-
ure S8).
In conclusion, we have successfully demonstrated the
rational design and preparation of a new type of UCNP, which
is capable of producing orthogonal and spectrally pure
emissions from core–shell NIR excitations at low power
density. In particular, the incorporation of two separate
excitation triggers allows for a higher degree of control than
achievable using previous systems. Furthermore, in contrast
to previously reported photoswitch systems, our system
excludes the necessity of high-energy UV/Vis, as well as
high-power-density NIR light, which attenuates any system
damage from multiphoton absorption and local heating.
Taken together, these results demonstrate that both our
novel designed UCNP and UCNP-SP-based NIR-driven two-
Figure 3. a) Illustration of two-way photoswitching of SP by using
Tm@Er UCNPs with dual NIR excitations. b) Tm3+ and Er3+ emissions
from Tm@Er nanoparticles under 808 nm and 980 nm excitations,
respectively, which overlap well with the absorption peaks of SP and
MC. The evolution of the UV/Vis absorption spectrum of the photo-
isomerization of SP and MC under 808 nm and 980 nm light irradi-
ation in the presence of Tm@Er UCNPs is shown. c) Kinetic monitor-
ing of the photoswitching reaction of SP and MC under 808 nm and
980 nm light irradiation with Tm@Er UCNPs by using the character-
istic absorbance of MC at 560 nm. The red dotted line shows the
kinetics of the reaction of MC to SP in the dark. d) Dual NIR light-
driven photoswitching of SP over many cycles in THF/methanol (9/1,
v/v) solution with 5 wt% of Tm@Er UCNPs and 10 mm of SP by
monitoring the absorbance of MC at 560 nm.
4
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Yin, Y. Chen, Y. Di, H. Chen, Y. Ma, S. Achilefu, Y. Gu, ACS
way photoswitching platform has great implications for
applications in materials science and biology studies.
Nano 2013, 7, 676; c) X. F. Qiao, J. C. Zhou, J. W. Xiao, Y. F.
Wang, L. D. Sun, C. H. Yan, Nanoscale 2012, 4, 4611; d) C. Wang,
L. Cheng, Y. M. Liu, X. J. Wang, X. X. Ma, Z. Y. Deng, Y. G. Li,
Z. Liu, Adv. Funct. Mater. 2013, 23, 3077.
Received: August 14, 2014
Revised: September 16, 2014
Published online: && &&, &&&&
[5] a) G. Chen, T. Y. Ohulchanskyy, R. Kumar, H. Agren, P. N.
Prasad, ACS Nano 2010, 4, 3163; b) N. N. Dong, M. Pedroni, F.
Piccinelli, G. Conti, A. Sbarbati, J. E. Ramirez-Hernandez, L. M.
Maestro, M. C. Iglesias-de la Cruz, F. Sanz-Rodriguez, A. Juar-
ranz, F. Chen, F. Vetrone, J. A. Capobianco, J. G. Sole, M.
Bettinelli, D. Jaque, A. Speghini, ACS Nano 2011, 5, 8665.
[6] a) J. C. Boyer, C. J. Carling, S. Y. Chua, D. Wilson, B. Johnsen, D.
Baillie, N. R. Branda, Chem. Eur. J. 2012, 18, 3122; b) J. C.
Boyer, C. J. Carling, B. D. Gates, N. R. Branda, J. Am. Chem.
Soc. 2010, 132, 15766; c) L. Wang, H. Dong, Y. Li, C. Xue, L. D.
Sun, C. H. Yan, Q. Li, J. Am. Chem. Soc. 2014, 136, 4480; d) Z.
Chen, L. Zhou, W. Bing, Z. Zhang, Z. Li, J. Ren, X. Qu, J. Am.
Chem. Soc. 2014, 136, 7498; e) F. Zhang, Q. Shi, Y. Zhang, Y. Shi,
K. Ding, D. Zhao, G. D. Stucky, Adv. Mater. 2011, 23, 3775; f) C.
Zhang, C. H. Xu, L. D. Sun, C. H. Yan, Chem. Asian J. 2012, 7,
2225; g) B. F. Zhang, M. Frigoli, F. Angiuli, F. Vetrone, J. A.
Capobianco, Chem. Commun. 2012, 48, 7244.
Keywords: core–shell nanoparticles · dual NIR excitations ·
.
nanoparticles · two-way photoswitching
[1] a) F. Auzel, Chem. Rev. 2004, 104, 139; b) S. W. Wu, G. Han, D. J.
Milliron, S. Aloni, V. Altoe, D. V. Talapin, B. E. Cohen, P. J.
Schuck, Proc. Natl. Acad. Sci. USA 2009, 106, 10917; c) F. Wang,
Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M.
Hong, X. Liu, Nature 2010, 463, 1061; d) F. Wang, R. Deng, J.
Wang, Q. X. Wang, Y. Han, H. Zhu, X. Chen, X. Liu, Nat. Mater.
2011, 10, 968; e) F. Wang, X. Liu, Chem. Soc. Rev. 2009, 38, 976;
f) J. Wang, R. Deng, M. A. MacDonald, B. Chen, J. Yuan, F.
Wang, D. Chi, T. S. A. Hor, P. Zhang, G. Liu, Y. Han, X. Liu, Nat.
Mater. 2014, 13, 157; g) M. Haase, H. Schafer, Angew. Chem. Int.
Ed. 2011, 50, 5808; Angew. Chem. 2011, 123, 5928.
[7] a) Q. Zhan, J. Qian, H. Liang, G. Somesfalean, D. Wang, S. He,
Z. Zhang, S. Andersson-Engels, ACS Nano 2011, 5, 3744; b) Z.
Jiang, M. Xu, F. Li, Y. Yu, J. Am. Chem. Soc. 2013, 135, 16446.
[8] a) J. Qiu, Y. Kawamoto, J. Appl. Phys. 2002, 91, 954; b) L. C.
Courrol, I. M. Ranieri, L. V. G. Tarelho, S. L. Baldochi, L.
Gomes, N. D. Vieira, J. Appl. Phys. 2005, 98, 113504; c) Y. L.
Huang, K. H. Jang, H. J. Seo, K. W. Jang, J. Appl. Phys 2006, 100,
083513; d) C. Koepke, K. Wisniewski, L. Sikorski, D. Piatkowski,
K. Kowalska, M. Naftaly, Opt. Mater. 2006, 28, 129; e) A. Lupei,
V. Lupei, C. Gheorghe, A. Ikesue, E. Osiac, Opt. Mater. 2009, 31,
744; f) Y. F. Wang, G. Y. Liu, L. D. Sun, J. W. Xiao, J. C. Zhou,
C. H. Yan, ACS Nano 2013, 7, 7200; g) X. Xie, N. Gao, R. Deng,
Q. Sun, Q. Xu, X. Liu, J. Am. Chem. Soc. 2013, 135, 12608; h) J.
Shen, G. Y. Chen, A. M. Vu, W. Fan, O. S. Bilsel, C. C. Chang, G.
Han, Adv. Opt. Mater. 2013, 1, 644; i) H. Wen, H. Zhu, X. Chen,
T. F. Hung, B. Wang, G. Zhu, S. F. Yu, F. Wang, Angew. Chem.
Int. Ed. 2013, 52, 13419; Angew. Chem. 2013, 125, 13661.
[9] Q. Su, S. Han, X. Xie, H. Zhu, H. Chen, C. Chen, R. Liu, X.
Chen, F. Wang, X. Liu, J. Am. Chem. Soc. 2012, 134, 20849.
[10] a) K. A. Abel, J. C. Boyer, C. M. Andrei, F. C. J. M. van Veggel,
J. Phys. Chem. Lett. 2011, 2, 185; b) C. Zhang, J. Y. Lee, ACS
Nano 2013, 7, 4393.
[2] a) L. L. Li, P. Wu, K. Hwang, Y. Lu, J. Am. Chem. Soc. 2013, 135,
2411; b) Y. Zhang, F. Zheng, T. Yang, W. Zhou, Y. Liu, N. Man,
L. Zhang, N. Jin, Q. Dou, Y. Zhang, Z. Li, L. P. Wen, Nat. Mater.
2012, 11, 817; c) R. Deng, X. Xie, M. Vendrell, Y. Chang, X. Liu,
J. Am. Chem. Soc. 2011, 133, 20168; d) H. Gorris, R. Ali, S. M.
Saleh, O. S. Wolfbeis, Adv. Mater. 2011, 23, 1652; e) S. H. Nam,
Y. M. Bae, Y. I. Park, J. H. Kim, H. M. Kim, J. S. Choi, K. T. Lee,
T. Hyeon, Y. D. Suh, Angew. Chem. Int. Ed. 2011, 50, 6093;
Angew. Chem. 2011, 123, 6217; f) X. Huang, S. Han, W. Huang,
X. Liu, Chem. Soc. Rev. 2013, 42, 173; g) H. H. Gorris, O. S.
Wolfbeis, Angew. Chem. Int. Ed. 2013, 52, 3584; Angew. Chem.
2013, 125, 3668; h) L. Sun, Y. Wang, C. Yan, Acc. Chem. Res.
2014, 47, 1001; i) Y. Yang, Q. Shao, R. Deng, C. Wang, X. Teng,
K. Cheng, Z. Cheng, L. Huang, Z. Liu, X. Liu, B. Xing, Angew.
Chem. Int. Ed. 2012, 51, 3125; Angew. Chem. 2012, 124, 3179;
j) Z. Gu, L. Yan, G. Tian, S. Li, Z. Chai, Y. Zhao, Adv. Mater.
2013, 25, 3758; k) Y. Liu, S. Zhou, D. Tu, Z. Chen, M. Huang, H.
Zhu, E. Ma, X. Chen, J. Am. Chem. Soc. 2012, 134, 15083; l) M.
Wang, C. C. Mi, W. X. Wang, C. H. Liu, Y. F. Wu, Z. R. Xu, C. B.
Mao, S. K. Xu, ACS Nano 2009, 3, 1580; m) Q. Liu, Y. Sun, T.
Yang, W. Feng, C. Li, F. Li, J. Am. Chem. Soc. 2011, 133, 17122;
n) J. Yang, D. Shen, X. Li, W. Li, Y. Fang, Y. Wei, C. Yao, B. Tu, F.
Zhang, D. Zhao, Chem. Eur. J. 2012, 18, 13642.
[11] J. Liu, W. Bu, L. Pan, J. Shi, Angew. Chem. Int. Ed. 2013, 52,
4375; Angew. Chem. 2013, 125, 4471.
[3] a) W. Li, J. Wang, J. Ren, X. Qu, J. Am. Chem. Soc. 2014, 136,
2248; b) B. Yan, J. C. Boyer, N. R. Branda, Y. Zhao, J. Am.
Chem. Soc. 2011, 133, 19714; c) J. Shen, G. Chen, T. Y.
Ohulchanskyy, S. J. Kesseli, S. Buchholz, Z. Li, P. N. Prasad, G.
Han, Small 2013, 9, 3213; d) C. J. Carling, F. Nourmohamma-
dian, J. C. Boyer, N. R. Branda, Angew. Chem. Int. Ed. 2010, 49,
3782; Angew. Chem. 2010, 122, 3870.
[12] a) R. Klajn, Chem. Soc. Rev. 2014, 43, 148; b) V. I. Minkin,
Chem. Rev. 2004, 104, 2751.
[13] a) T. Sakata, Y. L. Yan, G. Marriott, Proc. Natl. Acad. Sci. USA
2005, 102, 4759; b) Y. Ito, N. Sugimura, O. H. Kwon, Y. Imanishi,
Nat. Biotechnol. 1999, 17, 73; c) C. Beyer, H. A. Wagenknecht,
Synlett 2010, 1371; d) A. Kocer, M. Walko, W. Meijberg, B. L.
Feringa, Science 2005, 309, 755.
[4] a) N. M. Idris, M. K. Gnanasammandhan, J. Zhang, P. C. Ho, R.
Mahendran, Y. Zhang, Nat. Med. 2012, 18, 1580; b) S. Cui, D.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

.
Angewandte
Communications
Communications
Core–Shell Nanoparticles
Spiropyran photoswitching: A single-
crystal core–shell-structured upconver-
sion nanoparticle (Tm@Er) is capable of
orthogonal UV (365 nm) and visible
(545 nm) emissions in response to two
distinct NIR excitations at 808 and
J. Lai, Y. Zhang, N. Pasquale,
K.-B. Lee*
&&&& — &&&&
An Upconversion Nanoparticle with
Orthogonal Emissions Using Dual NIR
Excitations for Controlled Two-Way
Photoswitching
980 nm. It was applied in NIR-light-based
two-way photoswitching of spiropyran.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!