Simultaneous Observation of Visible Upconversion and Near-Infrared Downconversion in SrF2:Nd3+/Yb3+/Er3+ Nanocrystals and Their Application for Detecting Metal Ions under Dual-Wavelength Excitation.

In this work, a sequence of Nd3+, Yb3+, and Er3+ tridoped SrF2 nanocrystals (NCs) is synthesized by a hydrothermal method. Both the efficient near-infrared downconversion luminescence (DCL) and visible upconversion luminescence (UCL) of the Er3+ and Nd3+ ions are simultaneously observed and systematically demonstrated under dual-wavelength excitation (808 and 980 nm continuous-wave lasers). Subsequently, the SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs with the strongest luminescence were utilized for detecting the metal ion concentrations under 808 nm excitation. The results reveal that both the UCL and DCL gradually decrease as the metal ion concentrations increase, and high sensitivity is obtained for Cu2+ ions with a detection limit of 0.22 nM (∼650 nm) and 0.63 nM (∼976 nm). In addition, these SrF2:Nd3+/Yb3+/Er3+ NCs are further demonstrated to achieve a solid-state display under 980 nm excitation, exhibiting obvious "red" and "green" patterns by varying the doping rare earth ion concentrations.

Introduction

In recent years, lanthanide-based upconversion nanocrystals (NCs) have become a research hotspot due to their significant advantages of a long fluorescence lifetime, narrow spectral width, large anti-Stokes shift, and low biotoxicity, which have been widely applied in temperature sensing,[1−3] biological therapy,[4] micro-/nanolasers,[5,6] solar cells,[7] and other aspects.[8,9] Generally, the so-called upconversion luminescence (UCL) process is an important approach for obtaining efficient luminescence emission. UCL refers to the conversion of lower-energy near-infrared (NIR) photons into relatively high-energy visible light through continuous photon absorption and energy-transfer (ET) processes based on the activator–sensitizer pairs. Popularly, Er3+ ions are used as activators emitting UCL, while the Yb3+ and Nd3+ ions act as sensitizers absorbing the excitation energy. Meanwhile, Er3+ and Nd3+ ions have abundant energy levels, whose main emission peaks range from visible to NIR regions. Compared to Yb3+ ions, Nd3+ ions have a larger absorption cross-section (1.2 × 10–19 cm2) at 808 nm, which is about 10 times greater than that of Yb3+ ions at 980 nm.[10] In contrast, the water absorption coefficient of Nd3+ at 808 nm (0.02 cm–1) is much lower than that of Yb3+ ions at 980 nm (0.48 cm–1).[11] Thus, the Nd3+-sensitized NCs have been widely utilized in biological issues or aqueous environment applications.

The host material has the most important role in generating both UCL and downconversion luminescence (DCL). In general, the low phonon energy of the host lattice is likely to reduce the nonradiative (NR) transition rates, thus promoting emission efficiency. Fluorides have a relatively low phonon energy (∼350 cm–1), as well as strong ionic properties, high transmittance in a wide spectral range from ultraviolet to infrared, high damage threshold, a low refractive index, easy storage, and other superior properties,[12,13] which have been utilized for doping rare earth ions and applied in many practical fields. Among these fluorides, SrF2 is a promising host matrix with a cubic fluorspar structure and has attracted great attention.[14,15] Until now, many researchers have focused on investigating the UCL properties of NCs doped with either Yb3+ or Nd3+ ions as sensitizers.[16] However, there are still few reports on the simultaneous realization of UCL and DCL generation in lanthanide-doped NCs, especially for tridoping with Nd3+, Yb3+, and Er3+ ions. This can achieve 980 and 808 nm dual-wavelength excitation and further promote their applications in both aqueous and nonaqueous environments.[17]

With the development of society and industry, human beings have higher requirements for water quality. How to quickly and accurately detect the amount of metal ions in a water environment is particularly important. The Cu2+ ion is an essential trace element in the human body, and it is needed to catalyze many biochemical processes in the body, but too much intake will lead to cancer and other diseases.[18,19] The maximum allowance of Cu2+ ions in drinking water, given by the World Health Organization (WHO), is ∼31.5 μM.[20] At present, although the quantitative methods of metal ion detection include atomic absorption spectrometry, inductively coupled plasma mass spectrometry, or emission spectrometry, these methods are complicated and costly. In contrast, the optical sensors based on lanthanide-doped NCs are popularly employed to detect metal ions in the environment owing to their low cost and high sensitivity with fast response times.[21] Numerous reports have shown that lanthanide-doped NCs have good selectivity and high sensitivity for the detection of various metal ions.[22,23] Nevertheless, the present majority of sensors are operated by either singly detecting the UCL signal or under 980 nm excitation.

In this work, we have synthesized the Nd3+, Yb3+, and Er3+ tridoped SrF2 NCs through a hydrothermal approach. The structure of these SrF2 NCs was systematically characterized. Subsequently, we simultaneously investigated the visible UCL and NIR DCL properties in these NCs under the dual-wavelength excitation (both 808 and 980 nm). The dynamic properties, mechanism of population, and ET and emission processes are also demonstrated. Furthermore, the SrF2:Nd3+/Yb3+/Er3+(15/4/0.2 mol %) NCs were selected to act as high-sensitivity optical sensors for the detection of Cu2+ ions and other kinds of metal ions under 808 nm excitation. In addition, a potential solid-state display with different patterns is also demonstrated.

Experimental Section

Synthesis of Nanocrystals

The raw chemicals of SrCl2·6H2O (99.99%), YbCl3·6H2O (99.9%), NdCl3·6H2O (99.9%), ErCl3·6H2O (99.9%), Na3C6H5O7 (98%), and NH4F (98%) were all purchased from Aladdin (China). The procedure of the synthesis of NCs is similar to that in our previous report.[24] Taking SrF2:Nd3+/Yb3+/Er3+ (15/4/1 mol %) NCs as an example, 10 mL of solution of chloride salts (1.6 mmol SrCl2, 0.3 mmol NdCl3, 0.08 mmol YbCl3, and 0.02 mmol ErCl3) and 10 mL of aqueous solution of sodium citrate (1 M) were mixed under stirring for 1 h. Sequentially, 20 mL of aqueous solution of NH4F (1 M) was added into the above mixed solutions. The mixtures were thoroughly stirred for 30 min and then transferred into a 50 mL Teflon-lined autoclave and heated at 200 °C for 8 h. After the reaction, the autoclave was naturally cooled down to room temperature. The as-prepared SrF2 NCs were collected by centrifugation at 6000 rpm for 4 min, washed with ethanol and deionized water several times, and finally dried at 60 °C for 12 h.

Characterization

The morphology and size of the as-prepared SrF2 NCs were characterized by transmission electron microscopy (TEM). X-ray diffraction (XRD) patterns were measured using a powder diffractometer (Bruker D8 Advance) working in a Bragg–Brentano geometry ray with a copper anode X-ray source and a cooled solid-state detector. The visible and NIR luminescence spectra of SrF2:Nd3+/Yb3+/Er3+ NCs were measured using a fluorescence spectrophotometer (Zolix Omni-l3072i) coupled with an R928 photomultiplier tube or an InGaAs avalanche photodetector under the excitation of 808 and 980 nm lasers.

Metal Ion Detection Studies Using SrF2:Nd3+/Yb3+/Er3+ NCs

Taking the preparation of the Cu2+ ion solution (10–8 M) as an example, first, 0.5 mL of the SrF2:Nd3+/Yb3+/Er3+ NC dispersion solution (6 mg mL–1) was mixed with 0.15 mL of the Cu2+ ion solution (10–7 M). Then, 0.85 mL of anhydrous ethanol was added to the above mixed solutions, keeping the total volume to 1.5 mL. Finally, it was shaken well and set aside for further use. The configuration method of other Cu2+ concentrations and metal ion solutions is similar to the above procedure.

Solid-State Display Studies Using SrF2:Nd3+/Yb3+/Er3+ NCs

For the solid-state display studies, we engraved the different object patterns on the rubber seal. The as-prepared SrF2:Nd3+/Yb3+/Er3+ NCs coated the notch with a shallow layer of epoxy gum. Finally, we evenly and closely sprinkled a layer of the prepared NC powder and dried it before use.

Results and Discussion

Structure

Figure a–e shows the TEM images of SrF2:Nd3+/Yb3+ (15/4 mol %) NCs doped with different concentrations of Er3+ ions. It reveals that the prepared NCs are evenly distributed and mainly elliptic or square. Through the statistical analysis, the average size of the particle is approximately ∼55 nm, as shown in Figure f. The NCs are relatively uniform in size with small deviations. The results indicate that the doping of different concentrations of rare earth ions has little influence on the morphology and size of the SrF2 NCs.

Figure 1

TEM images of SrF2:Nd3+/Yb3+/Er3+(15/4/x mol %) NCs. (a) x = 0.2, (b) x = 1, (c) x = 2, (d) x = 5, and (e) x = 10. (f) Particle size distribution of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs.

Figure shows the XRD patterns of the SrF2:Nd3+/Yb3+ (15/4 mol %) NCs doped with different Er3+ ion concentrations. All the measured diffraction peaks of the samples can be well matched to the standard diffraction data of SrF2 (JCPDS card no. 06-0262), meaning that these as-prepared NCs all crystallize in a cubic structure with a space group of FM3̅m (225). Nevertheless, as the doping concentrations of Nd3+, Yb3+, and Er3+ ions increase, the XRD peak (∼26.77°) shifts slightly to a larger degree compared to that of the pure SrF2. The reason is that the radii of Yb3+ (0.86 Å), Nd3+ (0.99 Å), and Er3+ (0.88 Å) are all smaller than those of the Sr2+ ions (1.18 Å). When the bivalent Sr2+ ions are replaced by trivalent Yb3+ (Nd3+ or Er3+) ions, the distance between the lattice planes becomes smaller, thus leading to the diffraction angle increasing according to the Bragg equation.[25] No impurity phase is detected, indicating that the Nd3+, Yb3+, and Er3+ tridoped SrF2 NCs have been successfully synthesized.

Figure 2

XRD patterns of SrF2:Nd3+/Yb3+/Er3+ (15/4/x mol %) NCs doped with different Er3+ ion concentrations.

Photoluminescence

Figure shows both UCL and DCL spectra of Nd3+/Yb3+/Er3+ tridoped SrF2 NCs under the dual-wavelength excitation of 808 and 980 nm lasers. Efficient UCL and DCL can be detected. The pumping power intensity is 0.1 W cm–2 in our experiments to avoid or minimize the effects of laser-induced heat. Although the UCL and DCL can be efficiently observed at both 808 and 980 nm excitations at the same time, the total emission intensity obtained by an excitation of 808 nm is almost 10 times that by the 980 nm excitation under the same measurement conditions. This is because of the relatively larger absorption cross-section of Nd3+ at 808 nm compared to that of Yb3+ ions at 980 nm, as previously explained in the Introduction section.

Figure 3

Visible UCL and NIR DCL spectra of SrF2:Nd3+/Yb3+/Er3+ (15/4/x mol %) NCs doped with different Er3+ concentrations under the excitation of (a,b) 808 and (c,d) 980 nm, respectively.

As shown in Figure a, under the excitation of an 808 nm laser, typical UCL emissions of Er3+ can be clearly observed. The UCL emissions centered at 410, 521, 540, and 652 nm are ascribed to the 2H9/2 → 4I15/2, 4F7/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transition from Er3+ ions, respectively. In the NIR DCL spectra shown in Figure b, the peaks at 980 nm originate from the 2F5/2 → 2F7/2 transitions from Yb3+ ions, while the peaks at 867, 1056, and 1330 nm originate from the 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, and 4F3/2 → 4I13/2 transitions from Nd3+ ions, respectively. The spectral intensity of the NCs gradually decreases with the increase of Er3+ concentration, and 0.2% Er3+ is the optimal concentration for achieving the strongest luminescence output. Under the excitation of a 980 nm laser, the shape of the spectra and the concentration-dependent luminescence are the same as those of the NCs excited under 808 nm,[26] as shown in Figure c,d.

To clarify the ET mechanism of SrF2:Nd3+/Yb3+/Er3+ NCs excited at 808 and 980 nm, Figure displays the energy level diagram containing the ET processes, excited-state absorption (ESA), cross-relaxation (CR), and NR transition. Under the excitation of 808 nm, when the NCs are codoped with Nd3+ and Yb3+ ions, Nd3+ ions can be populated through the 4I9/2 → 4F5/2 transition by absorbing 808 nm photons.[27] Subsequently, the electrons in the 4F3/2 state will transition to the 4I9/2, 4I11/2, and 4I13/2 states, thereby emitting the NIR light at 867, 1056, and 1330 nm, respectively. Sectional electrons in the 4F5/2 state reach the 2G9/2 state by the ESA process and then populate the excited states of 4G7/2, 4G5/2, and 4F13/2 by NR transitions, generating the UCL signals at 542, 586, and 667 nm, respectively. In addition, the 4F5/2 state of Nd3+ can populate the 2F5/2 state of Yb3+ through the ET process, thus producing 976 nm NIR DCL. When further introduced with Er3+ ions, the Nd3+ ions could populate the 2F5/2 state of adjacent Yb3+ by ET processes and sequentially transfer energy to the 4I11/2, 4F7/2, and 2H9/2 states of Er3+ ions through several successive ET processes.[28,29] In addition, besides the population process of Nd3+ → Yb3+ → Er3+ mentioned above, Nd3+ can directly populate the 4I9/2 and 2H9/2 states of Er3+ ions through the ET process (Nd3+ → Er3+) between Nd3+ and Yb3+ ions. The efficient population of the high-energy states of Er3+ can relax them to the corresponding excited states through the NR transition, such as 2H9/2, 2H11/2, 4S3/2, and 4F9/2 states, thus generating the UCL centered at 410, 521, 540, and 650 nm, respectively.

Figure 4

Energy level diagram for SrF2:Nd3+/Yb3+/Er3+ NCs excited at 808 and 980 nm, respectively. The corresponding processes of ET, ESA, and CR; NR transitions; UCL and DCL channels are also provided.

Instead of Nd3+ ions absorbing 808 nm photons, the Yb3+ ions can absorb 980 nm photons directly to make the transition of 2F7/2 → 2F5/2.[30,31] On the one hand, Yb3+ as the sensitizer will transfer energy to the adjacent Nd3+ ions through ET processes, populating the 2G9/2 and 4F3/2 states of Nd3+ ions. Especially, the CR1 process can significantly populate the 4F5/2 state. This can efficiently generate the 804 nm UCL compared to pumped at 808 nm laser. In addition, the CR2 process of 4I9/2 (Nd3+) + 4F9/2 (Er3+) → 4I15/2 (Er3+) + 4F9/2 (Nd3+) will promote the population of the 4F9/2 state of Nd3+ and then radiate to the 4H7/2 state, achieving the 750 nm (4H7/2 → 4I9/2) emission.[32] On the other hand, Yb3+ can also transfer energy to Er3+ ions by ET processes to populate the 4I11/2, 4F9/2, 4F7/2, and 2H9/2 states of Er3+, thus leading to the typical UCL of Er3+.

Figure a,c shows the UCL spectra of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs excited by 808 and 980 nm lasers with different pumping power densities. All the UCL intensities increase gradually with the pumping power density. It is well known that the UCL intensity and pumping power density follow the relationship, I ∝ P, that is, the UCL intensity (I) is proportional to the pumping power density (P), and n is the number of photons absorbed for each emitted photon. Figure b,d further display the dependence of UCL intensity on the pumping power density. The slope of the fitted curve for 410 nm is greater than 2, indicating that it originates from the three-photon absorption process. The slopes of the other UCL curves are between 1 and 2, revealing that these observed UCLs originate from the two-photon absorption process. The results match well with the discussed UC emission processes in Figure .

Figure 5

Spectra of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs excited by (a) 808 and (c) 980 nm lasers with different pumping power densities. Double-logarithmic plots of luminescent intensity vs different pumping power densities under (b) 808 and (d) 980 nm.

Metal Ion Concentration Detection

Having demonstrated the efficient UCL and DCL of the as-prepared NCs, here, we select the SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs as an optical sensor to detect the metal ion. Figure a gives the spectra of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs dispersed in different concentrations of Cu2+ ion solutions under the excitation of an 808 nm laser. With the increase of Cu2+ ion concentrations from 1 to 10 nM, the peak positions of the UCL and DCL remain unchanged. However, both the UCL and DCL gradually decrease when increasing the Cu2+ concentrations. The luminescence inhibition effect of the NCs becomes more obvious at relatively high Cu2+ concentrations. This is possibly caused by the addition of Cu2+ ions which inhibit the process of ET between the rare earth ions doped in the NCs.[33−35] This luminescence quenching effect can be utilized for optical sensor detection of the metal ion concentrations. Generally, the Stern–Volmer equation is used to describe the photoluminescence quenching process (dynamic/static), which is defined as follows[36]where I0 and I represent the luminescence intensity in the absence and presence of Cu2+ ions, respectively, KSV is the dynamic quenching constant (Stern–Volmer quenching constant), and Q is the Cu2+ concentration of the solution. Based on the spectral data in Figure a, we have calculated the ratios of 652 nm visible emission and 976 nm NIR light. Figure b,c shows the corresponding Stern–Volmer plots as a function of the Cu2+ concentrations. According to the plot, a linear relationship can be obtained between the luminescence ratio and the Cu2+ ion concentration for both 652 nm (R2 = 0.984) and 976 nm (R2 = 0.997). The limit of detection (LOD) can be further calculated using the following formula[37,38]where σ is the standard deviation of the blank experiment and S is the slope of the calibration plot. In our measurements, σ is about 3.77 × 10–4, which is calculated after several measurements of the original sample without Cu2+ ion doping. According to the obtained parameters, the detection limit value was therefore calculated to be 0.22 nM (652 nm) and 0.63 nM (976 nm), respectively. This probed value is extremely higher than the maximum Cu2+ ion concentration (∼31.5 μM) permitted in drinking water given by the WHO.[20] Therefore, these NCs can be applied for detecting the Cu2+ ions in the drinking water for quality monitoring. Compared with other NCs, these NCs have a better detection accuracy for Cu2+ ion concentration detection, which is summarized in Table .

Figure 6

(a) DCL and UCL intensity of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs as a function of the Cu2+ ion concentrations. The Stern–Volmer plot as a function of Cu2+ concentration for SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs measured at (b) visible UCL (652 nm) and (c) NIR DCL (976 nm), respectively. The excitation wavelength is 808 nm.

Table 1

LOD and Linearity Range Comparison of Various Samples Applied for the Detection of Cu2+ Ions

samples	λex (nm)	λem (nm)	concentration detection range	LOD	reference
CuInS2/ZnS	450	695	75–750 nM	63 nM	(21)
Cit/CaF2:Ce3+/Tb3+	302	543	2–20 μM	10.2 μM	(23)
SrF2:Ce3+/Tb3+	290	544	1–10 nM	2.2 nM	(36)
CdTe/Fe3O4	400	575	2–800 μM	1.8 μM	(39)
KZnF3:Eu3+	394	614	10–100 μM	0.48 μM	(40)
BaF2:Ce3+/Tb3+	287	545	20–100 μM		(41)
SrF2:Nd3+/Yb3+/Er3+	808	652	1–10 nM	0.22 nM	this work
SrF2:Nd3+/Yb3+/Er3+	808	976	1–10 nM	0.63 nM	this work

Figure a,b displays the decay curve of the emission peak at 652 and 976 nm with changing the Cu2+ concentrations from 1 to 10 nM in the SrF2:Nd3+/Yb3+/Er3+ NC sample under 808 nm excitation. Meanwhile, to clearly compare the lifetime variations, Figure c,d further shows the dependence of both the visible UCL and NIR DCL lifetimes on the Cu2+ concentrations. A single exponential function has been used to fit the lifetime of the fluorescence decay curves. It can be seen that both the lifetimes of 652 and 976 nm gradually decrease with the increase of Cu2+ concentrations. The lifetime of the transition 4F9/2 → 4I15/2 (652 nm) of Er3+ ions drops from 275.15 to 220.89 μs as well as that of the transition 2F5/2 → 2F7/2 (976 nm) of Yb3+ ions decreases from 338.43 to 306.72 μs.

Figure 7

Lifetime of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs dispersed in different concentrations of Cu2+ ions at the (a) visible UCL (652 nm) and (b) NIR DCL (976 nm). Dependence of (c) visible UCL (652 nm) and (d) NIR DCL (976 nm) lifetimes of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs on the Cu2+ ion concentrations. The excitation wavelengths are all 808 nm.

After investigating the detection of Cu2+ ion concentrations, we tried to detect other metal ions. Figure a shows the emission spectra of SrF2:Nd3+/Yb3+/Er3+ NCs dispersed in solutions with different kinds of metal ions at the same concentrations (10 nM) under 808 nm excitation. To clearly quantitatively determine the influence of various metal ions on the luminescence property, the emission intensity of the sample without any metal ion is taken as the reference value “1”, as shown in Figure b. The results manifest that Li+ ions have the least inhibitory effect on the UCL and DCL, while Ca2+ and Zn2+ ions have the strongest quenching effects. Generally, numerous efforts have been made to detect metal ions, including atomic absorption spectroscopy, electrochemical sensing, X-ray fluorescence, and optical spectroscopy detection. The possible quenching principle and explanation, such as the photoinduced electron transfer, intramolecular charge transfer, and fluorescence resonance ET, have also been widely discussed in some previous reports.[42−45]

Figure 8

(a) Spectra of SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs dispersed in different metal ions with the same concentration (10 nM) excited under an 808 nm laser. (b) Normalized spectral intensity of different kinds of metal ions (the luminescence intensity for the NCs without any metal ion is normalized as the reference value “1”).

Solid-State Imaging Display

To further visualize the emission spectra of our samples and expand the application of the NCs in solid-state imaging displays and anti-counterfeiting identification, we used the NC embedding in the engraving vivid pictures under 980 nm excitation, as shown in Figure . Picture I (basketball) is drawn with SrF2:Nd3+/Yb3+/Er3+ (15/4/0.5 mol %) NCs. It is noted that the excitation of different samples under the same wavelength laser can get different imaging colors. Bright and clear “red” patterns can be observed in pictures I (basketball), II (guitar), and III (sailboat) achieved with the SrF2:Nd3+/Yb3+/Er3+ (15/4/0.5 mol %), SrF2:Nd3+/Yb3+/Er3+ (15/4/1 mol %), and SrF2:Nd3+/Yb3+/Er3+ (15/4/2 mol %) NCs, respectively. Moreover, “green” patterns can be detected in pictures IV (umbrella) and V (flowers) when sealed with SrF2:Nd3+/Yb3+/Er3+ (15/4/10 mol %) and SrF2:Nd3+/Yb3+/Er3+ (10/2/1 mol %) NCs. Therefore, we fully believe that a specific color light output can be achieved through doping with different Nd3+, Yb3+, and Er3+ concentrations, which will greatly promote the development of solid-state display research.

Figure 9

(a) Original pictures of a basketball (I), guitar (II), sailboat (III), umbrella (IV), and flower (V). (b) Displayed “red” and “green” patterns drawn with the prepared SrF2 NCs doped with different Nd3+, Yb3+, and Er3+ ions under 980 nm laser illumination. The drawn NCs are (I) SrF2:Nd3+/Yb3+/Er3+ (15/4/0.5 mol %), (II) SrF2:Nd3+/Yb3+/Er3+ (15/4/1 mol %), (III) SrF2:Nd3+/Yb3+/Er3+ (15/4/2 mol %), (IV) SrF2:Nd3+/Yb3+/Er3+ (15/4/10 mol %), and (V) SrF2:Nd3+/Yb3+/Er3+ (10/2/1 mol %).

Conclusions

In summary, we have fabricated the Nd3+, Yb3+, and Er3+ tridoped SrF2 NCs with an average size of 55 nm by a hydrothermal method. Subsequently, their NIR DCL and visible UCL properties are systematically characterized under dual-wavelength excitation (both 808 and 980 nm lasers). The results verify that these NCs can simultaneously efficiently exhibit both the NIR DCL and UCL signals. Based on the spectra under dual-wavelength excitation, the mechanism of the DCL and UCL and populations of these SrF2:Nd3+/Yb3+/Er3+ NCs are demonstrated. Furthermore, the SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs are further utilized for detecting the Cu2+ ions, and the probe accuracy is on the nanomolar concentration scale (0.22 nM at 652 and 0.63 nM at 976 nm). It also reveals that these NCs can be applied for detection of other common metal ion concentration. In addition, we embedded the NCs doped with different Nd3+, Yb3+, and Er3+ ions into the engraving structure pictures and achieved obvious “red” and “green” patterns for a solid-state imaging display. We firmly believe that these NCs have great potential applications in the detection of metal ion concentration and solid-state displays.