High selectivity up-converted fluorescence turn-on probe for Zn²⁺ based on PAMAM hydroxy-naphthalene Schiff-bases (C=N) half-organic quantum dots

Abstract

The dendrimer PNS-G0 with Schiff-base imines (C=N) realizes highly selective and common fluorescence (λ_ex = 400 nm) or up-converted fluorescence (λ_ex = 800 nm) turn-on effect to qualitatively and quantitatively detect zinc ions (Zn²⁺). The PNS-G0 + Zn²⁺ complex (C=N_Zn_O) fluorescence emitting parts were firstly proposed as half-organic quantum dots (HOQDs). The HOQDs probe PNS-G0 for Zn²⁺ by the coordination method based on imine isomerization (inhibition) mechanism, which has potential applications in biological imaging, analytical chemistry, and optical physics areas.

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1. Introduction

Zinc is an important element in biological tissues, for many enzymes and proteins include the zinc ion (Zn²⁺).^1,2 Therefore, the qualitative and quantitative analysis for Zn²⁺ in tissues and the detection of Zn²⁺ positions are important in the study of biological processes. Fluorescent probe molecules are simple, sensitive, fast, and efficient tools for detecting Zn²⁺ ions.³ The two-photon absorption induced up-converted fluorescence probe for Zn²⁺ has been a topic of interest in biology, chemistry, and even physics areas.⁴

Several probe mechanisms have been reviewed,⁵ such as photoinduced electron transfer (PET),⁶ excimer/exciplex formation,⁷ intramolecular charge transfer (ICT),⁸ metal–ligand charge transfer (MLCT),⁹ fluorescence resonance energy transfer (FRET),¹⁰ twisted intramolecular charge transfer (TICT),¹¹ electronic energy transfer (EET),¹² aggregation-induced emission (AIE),¹³ and excited-state intramolecular proton transfer (ESIPT).¹⁴ Recently, the Schiff-base imine (C=N) isomerization phenomena were investigated and have been used in the design and synthesis of a new type of probe.¹⁵ The Schiff-base imine (C=N) exhibits no or weak fluorescence for the isomerization phenomena. If the imine isomerization is inhibited by complexing with metal ions through coordination bonds, then the imine exhibits fluorescence emission.⁵ These C=N isomerization-based probes can realize a fluorescence turn-on effect in detecting various metal ions (e.g. Mg²⁺, Cu²⁺, Cd²⁺).¹⁶

Based on this strategy, a probe for Zn²⁺ ions was developed by synthesizing the dendrimer PNS-G0 (Scheme 1) with imine (C=N) groups from poly-(amido-amine) generation zero (PAMAM G0) and 2-hydroxy-1-naphthaldehyde through the Schiff base reaction. PNS-G0 has a PAMAM-G0 center core with many amines (tertiary amine, amide, and imine). Furthermore, PAMAM has intramolecular holes between flexible chains, which can complex with metal ions and package nanoparticles or small molecules, and even realize drug delivery. The PAMAM can emit fluorescence after storage or oxidation in air,¹⁷ which has elicited interest in related areas such as fluorescence probe design. The end group of PNS-G0 is 2-hydroxy-1-naphthaldehyde. The hydroxyl was reserved for forming the coordination bond. The aldehyde can react with the amine of PAMAM by Schiff base condensation reaction, which can form the imine (C=N) group for use in the C=N isomerization mechanism.

Scheme 1 The structures of PNS-G0 and its complex with Zn²⁺.

The semiconductor nanoparticles known as quantum dots (QDs),¹⁸ are characterized by large Stokes shifts, broad absorption bands, and narrow, size-dependent emission bands without a significant red tail. The multiple color QDs can be excited using a single laser excitation wavelength.¹⁹ The size-dependent emission of QDs is the result of a quantum confinement effect (QCE). The QCE occurs when the size of the exciton (exciton Bohr radius, B) exceeds the physical size of the semiconductor nanocrystal (D).²⁰ Semiconductor nanocrystals exhibit particularly interesting properties when the exciton is strongly confined (D < 2B);²¹ they outperform fluorescent dyes in terms of brightness and photo stability.²² Inorganic QDs composited with organic molecules form a new type of QDs, as half-organic quantum dots (HOQDs) were firstly named. HOQDs have been involved in recent research, such as cysteine thiol QDs²³ and PAMAM thiol modified QDs,²⁴ as well as other half-organic part QDs.²⁵ In this paper, the imine_Zn_hydroxyl (C=N_Zn_O) complex HOQDs are discussed.

2. Results and discussions

2.1. Selectivity of probing Zn²⁺

Fig. 1 shows the photographs of PNS-G0 and its complex with several metal ions in DMSO solutions. From Fig. 1, the PNS-G0 gave weak emission under UV light. PNS-G0 gave strong blue fluorescence after complexing with Zn²⁺. PNS-G0 complexed with other metal ions (Ca²⁺, Mg²⁺, Pb²⁺, and Cd²⁺) gave weak fluorescence. Fig. 1 shows the fluorescence selectivity of PNS-G0 for the Zn²⁺ probe as distinguished by the naked eye.

Fig. 1 Photographs of the probe PNS-G0 and its complex with different metal ions (Zn²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺) in DMSO (5 × 10⁻⁵ mol L⁻¹). The upper photograph was in room light; the lower photograph was in UV light (365 nm).

Fig. 2 gives the UV-vis absorption spectra of PNS-G0 and the PNS-G0 complex with Zn²⁺ and other metal ions. The main absorbance peaks were at 250 nm, 300 nm, and 400 nm. Compared with the UV-vis absorption spectra of the PNS-G0 and PNS-G0 complex, the Zn²⁺ ion complex has a higher UV-vis absorbance peak at 250 nm and a lower UV-vis absorbance peak at 300 nm than those of other metal ion complexes. The Zn²⁺ ion complex UV-vis absorbance peaks have 380 nm peaks and show a blue shift compared with other metal ion complexes. The UV-vis absorption spectra show different spectra for the PNS-G0 + Zn²⁺ complex compared with other metal ions. It can be seen that the Zn²⁺ complex with PNS-G0 changes the n–π* and π–π* transition absorbance, which shows the unique absorbance characteristic of the Zn²⁺ + PNS-G0 complex, different from the UV-vis absorption spectra of PNS-G0 and the other metal ion complexes.

Fig. 2 The UV-vis absorption spectra of PNS-G0 (5 × 10⁻⁵ mol L⁻¹) and PNS-G0 complex with different metal ions (Zn²⁺, Cd²⁺, Mg²⁺, Ca²⁺) (1 × 10⁻⁴ mol L⁻¹).

The fluorescence emission spectra in Fig. 3 show that the PNS-G0 gave ten times fluorescence enhancement after complexing with Zn²⁺, while other metal ion complexes gave very weak fluorescence emission. Employing the sulfuric acid quinine fluorescence reference method test, the fluorescence quantum yield Φ of PNS-G0 and its metal ion complexes gave Φ_PNS-G0 = 0.06; Φ_{PNS-G0+Ca²⁺} = 0.01; Φ_{PNS-G0+Mg²⁺} = 0.04; Φ_{PNS-G0+Pb²⁺} = 0.01; Φ_{PNS-G0+Cd²⁺} = 0.02; Φ_{PNS-G0+Zn²⁺} = 0.73, which show that the PNS-G0 complex with Zn²⁺ gave the highest Φ, about ten times than that of others. Fig. 4 shows the PNS-G0 complexes in mixed solvent H₂O–DMSO = 99 : 1, and the Zn²⁺ complex has higher emission intensity than other metal ion complexes. The introduction of water to the solvent can enhance biocompatibility, and PNS-G0 shows good fluorescence selectivity in this mixed solvent. It can be seen from Fig. 1–4 that the PNS-G0 probe realized fluorescence enhanced and fluorescence turn-on effect on detecting Zn²⁺ and showed high selectivity by comparing with other metal ions.

Fig. 3 The fluorescence emission of the PNS-G0 probe (5 × 10⁻⁵ mol L⁻¹) and its complexes with different metal ions (Zn²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Fe³⁺, Fe²⁺, Ni²⁺, Cu²⁺, Co²⁺, Cr³⁺, Ag⁺, H⁺, NH₄⁺, Na⁺, K⁺) (1 × 10⁻⁴ mol L⁻¹) in DMSO (excited under 400 nm wavelength light).

Fig. 4 The fluorescence emission of probe PNS-G0 (1 × 10⁻⁶ mol L⁻¹) and its complexes with different metal ions (Zn²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺) (1 × 10⁻⁴ mol L⁻¹) in mixed solvent H₂O–DMSO = 99 : 1 (excited under 400 nm wavelength light).

2.2 Quantitative probing of sZn²⁺

The UV-vis absorption spectra of PNS-G0 and its complex with different concentrations of Zn²⁺ in DMSO solvent are shown in Fig. 5. The UV-vis absorbance peaks become higher at 250 nm and lower at 300 nm along with increasing Zn²⁺ concentration. It was noticed that the UV-vis absorbance peaks at about 400 nm gave a blue shift along with the increasing Zn²⁺ concentration, which shows the Zn²⁺ influence on the π–π* conjugated imine-naphthalene absorbance after complexing with PNS-G0.

Fig. 5 The UV-vis absorption spectra of PNS-G0 (1 × 10⁻⁴ mol L⁻¹) and PNS-G0 complex with different concentrations of [Zn²⁺] (mol L⁻¹).

Fig. 6 shows the fluorescence .emission spectra of PNS-G0 + Zn²⁺ complex with different concentrations of Zn²⁺ in DMSO. The PNS-G0 was kept at 5 × 10⁻⁵ mol L⁻¹ and added into Zn²⁺ DMSO solution with concentrations from 1 × 10⁻⁸ mol L⁻¹ to 1 × 10⁻³ mol L⁻¹. The fluorescence increased along with the increasing [Zn²⁺] concentration.

Fig. 6 The fluorescence emission spectra of PNS-G0 (5 × 10⁻⁵ mol L⁻¹) and PNS-G0 complex with different concentrations [Zn²⁺] in DMSO.

PNS-G0 can qualitatively detect Zn²⁺ and also can quantitatively detect the Zn²⁺, as shown in Fig. 7. The inset graph in Fig. 7 shows the good linear relationship of the PNS-G0 probing the [Zn²⁺] (range from 1 × 10⁻⁸ mol L⁻¹ to 5 × 10⁻⁶ mol L⁻¹). From the data in Fig. 5, the approximate binding constant of the single branch of PNS-G0 was K₁ ≈ 2 × 10³ M⁻¹ by the equation²⁶ ΔA/L = (([Dt]K₁Δε[M])/(1 + K₁[M])). The fluorescence spectra in Fig. 8 shows the PNS-G0 qualitatively detected the [Zn²⁺] in mixed solvent H₂O–DMSO = 99 : 1. The 1/(F − F_min) vs. 1/[Zn²⁺]^26c have good linear relationship. It can be seen from Fig. 5–8 that the probe PNS-G0 realized qualitative detection of the Zn²⁺ by the fluorescence method. The use of organic solvent and the nearly two times enhanced selective signal in water–DMSO mixed solvents show the dyes are inconvenient for biological imaging and need further modification.

Fig. 7 The fluorescence emission spectra of PNS-G0 (5 × 10⁻⁵ mol L⁻¹) added with Zn²⁺ (1 × 10⁻⁸ mol L⁻¹ ∼5 × 10⁻⁶ mol L⁻¹) in DMSO excited under 400 nm wavelength light inset: 1/(F − F_min) vs. 1/[Zn²⁺], F is the fluorescent emission spectra integrated area; F_min is the minimum [Zn²⁺]-added sample's fluorescence spectra integrated area.

Fig. 8 The fluorescence emission spectra (top) of PNS-G0 (1 × 10⁻⁶ mol L⁻¹) and PNS-G0 complex with different concentrations of [Zn²⁺] in mixed solvent H₂O–DMSO = 99 : 1 (excited under 400 nm wavelength light). 1/(F − F_min) vs. 1/[Zn²⁺] (bottom): F is fluorescent emission spectra integrated area; F_min is the minimum [Zn²⁺]-added sample's fluorescence spectra integrated area.

2.3 Up-converted fluorescence spectra

The up-converted fluorescence (anti-Stokes emissions) can realize long wavelength excitation and short wavelength emission.²⁷ After the advent of lasers, the two- and even three-photon absorption-induced frequency up-converted fluorescence in organic dye materials could be observed by using pulsed laser excitation.²⁸ The long excitation wavelength of up-converted fluorescence can reduce optical damage, achieve deep penetration, dark field imaging, high resolution three-dimensional display, and other advantages of probes used in biological tissues.²⁹

Fig. 9 shows the emission peaks at 535 nm as a result of excitation under 800 nm wavelength light, which is attributed to the two-photon mechanism, up-converted fluorescence. The up-converted fluorescence emission spectra of PNS-G0 and complex PNS-G0 + Zn²⁺ realize turn-on effect in DMSO (insert in Fig. 9). The up-converted fluorescence properties of PNS-G0 excited under 800 nm also can qualitatively and quantitatively detect the Zn²⁺, as shown in Fig. 9. The introduction of water mixed with 1% DMSO as solvent extends the application of PNS-G0 in the imaging of cells or tissue.

Fig. 9 Up-converted fluorescence emission spectra of PNS-G0 (1 × 10⁻⁶ mol L⁻¹) and complex with different concentrations of Zn²⁺ in solvent H₂O–DMSO = 99 : 1 (top figure) and up-converted fluorescence spectra of PNS-G0 compared complex with Zn²⁺ in DMSO (bottom figure). Inset: 1/(F − F_min) vs. 1/[Zn²⁺] figure, F is fluorescent intensity, (R² = 0.97). The excitation wavelength is 800 nm. F_min is the minimum [Zn²⁺]-added sample's fluorescence intensity.

Fig. 10 shows the up-converted fluorescence emission spectra of the PNS-G0 probe and its complexes with different metal ions (Zn²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺) in mixed solvent H₂O–DMSO = 99 : 1 excited under 800 nm wavelength light. Fig. 10 shows that the PNS-G0 + Zn²⁺ complex has higher fluorescence emission peaks than that of PNS-G0 and other metal ion complexes excited at 800 nm wavelength light. It can be seen from Fig. 10 that the probe PNS-G0 realized nearly two-times selective detection of Zn²⁺ compared with other metal ions by up-conversion of the fluorescence spectra.

Fig. 10 The up-converted fluorescence emission spectra of probe PNS-G0 (1 × 10⁻⁶ mol L⁻¹) and its complex with different metal ions (Zn²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺) (1 × 10⁻⁴ mol L⁻¹) in mixed solvent H₂O–DMSO = 99 : 1 (excited under 800 nm wavelength light).

The spectra in Fig. 9 and 10 show the up-converted fluorescence under common linear light source. The experiments were taken by FluoroMax-4 spectrofluorometer using the excitation source xenon, continuous output, ozone-free lamp (150 W) at room temperature (∼20 °C), which replaced the laser instruments. The emission phenomena of linear light source-excited organic molecules, giving up-converted fluorescence, were firstly tested. The FluoroMax-4 spectrofluorometer is the common fluorescence testing instrument. The light source was linear, which is different from the laser source. The linear source-excited, up-converted fluorescence organic molecules included PNS-G0 + Zn²⁺ dendrimers, which might be of interest in chemistry, physics, and biology areas.

Up-converted fluorescence has three mechanisms:³⁰ the first is second-harmonic generation, the second is two-photon absorption, and the third is the photon avalanche³¹ process. The UV-vis absorption spectra in Fig. 2 and 5 show no linear absorbance at 800 nm, indicating the existence of nonlinear absorbance that produced the fluorescence emission under 800 nm excitation wavelength. Fig. 9 shows that the emission peaks at about 535 nm are attributed to two-photon absorption induced up-converted fluorescence mechanism.

Fig. 9, top image, shows the PNS-G0 complex with Zn²⁺ emission fluorescence at about 535 nm in mixed solvent H₂O–DMSO = 99 : 1 and emission at 450 nm (bottom image) in DMSO solvent (excited under 800 nm wavelength light), which represents about an 85 nm shift. The most probable explanation is the influence of solvent. The solvent effect parameters, including polarity, dielectric constant (ε₀), refractive index (n), and orientation polarizability (Δf), influence the PNS-G0 complex fluorescence emission. The different dipole–dipole actions of the solvent provide different environments for the PNS-G0, including changed excited-states and dipole moments, to give different fluorescence emission peaks. The solvent also transfers energy to the PNS-G0 under the external light excitation, which might influence the fluorescence emission positions. The influence of different solvent effects of water and DMSO on the PNS-G0 + Zn²⁺ complex at 800 nm excitation wavelength may cause the 85 nm shifts of up-converted fluorescence emission peaks shown in Fig. 9 (top image).

2.4 Imine (C=N) isomerization mechanism

Consider the mechanism illustrated in Fig. 11 showing the Schiff base imine (C=N) isomerization with the formation of the single/double bond resonance structures, which release the energy of excited molecular states through non-radiative processes at the microscopic scale, and give no or weak fluorescence at the macroscopic scale. However, if the C=N isomerization is inhibited by complexing with metal ions through coordination bonds, then the imine C=N can enhance fluorescence emission ratios. PNS-G0 with imine C=N bonds were used in this mechanism to probe for Zn²⁺. The probe PNS-G0 gave weak fluorescence before complexing with Zn²⁺. When detecting the Zn²⁺, the imine (C=N), hydroxyl (HO–), and amines of PAMAM form coordination bonds with the ions. Then the C=N_Zn_O parts with naphthalene give strong fluorescence emission. The imine (C=N) is locked by coordination bonds with Zn²⁺, and then the naphthalene (as donor) and imine (C=N) (as acceptor) form a D–A (push–pull type) dipole emission unit. This dipole unit is a coplanar conjugated group and can enhance nonlinear optical properties, such as two-photon, up-converted fluorescence emission. These interactions cause the up-converted fluorescence emission of PNS-G0 and the turn-on effect when probing Zn²⁺ ions.

Fig. 11 Schiff-base imine (C=N) isomerization inhibition mechanism.

2.5 Half-organic quantum dots (HOQDs)

PNS-G0 matched Zn²⁺ gave strong fluorescence emission, which was different from other metal ions complexed with PNS-G0. The fluorescence spectra in Fig. 3, 4, and 10 prove that PNS-G0 has good selectivity for Zn²⁺. These show the PNS-G0 complexes with Zn²⁺ to form half-inorganic (Zn ions) + half-organic (hydroxyl-naphthalene imine) structures, which are unique fluorescence emission groups that are different from other metal ions complexed with PNS-G0.

Fig. 4, 8, and 9 show that the sharp emission peaks in mixed solvent H₂O–DMSO = 99 : 1 are different from the spectra in DMSO solvent. Fig. 4 and 8 show peaks ranging from 455 nm to 475 nm, about 20 nm wide. The PNS-G0 gave enhanced emission at these narrow ranges. For example, Fig. 4 shows one sharp peak from 12 500 to 24 000 a.u., an increase of 11 500 a.u. The excitation wavelength also influences the sharp shape of emission peaks. At the excitation wavelength of 400 nm (Fig. 4 and 8), the complex gave sharp peaks with range about 20 nm (455 nm to 475 nm); at the excitation wavelength of 800 nm (Fig. 9), the complex gave sharp peaks with a range of about 10 nm (530 nm to 540 nm).

The PNS-G0 + Zn²⁺ complex (Fig. 12) has a ZnO unit. ZnO is a traditional, typical quantum dot.³² The sharp type of emission peaks also appeared in the photoluminescence spectrum of the ZnO–PAMAM–G3 nanocomposite dispersed in water after excitation at 350 nm, at room temperature.³³ These sharp peaks may be associated with PAMAM, Zn element, water solvent, the excitation wavelength, and even the nano states of molecules or particles. The narrow emission peaks of this PNS-G0 + Zn²⁺ complex are similar with the narrow emission band characteristics of fluorescence quantum dots (QDs). PNS-G0 + Zn²⁺ has sharp peaks in the fluorescence emission spectra—with narrow emission peaks—and the PNS-G0 complex with Zn²⁺ has inorganic metal ions and organic hydroxyl + imine ligands. So this PNS-G0 + Zn²⁺ complex has organic conjugated parts and ZnO QDs parts, which are firstly called half-organic quantum dots (HOQDs).

Fig. 12 The half-organic quantum dots (HOQDs) of C=N_Zn_O.

The HOQDs gave different sharp peaks shapes and different emission positions under different excitation wavelengths or solvents. Solvent effect and the environment of complex molecules distributed in solution co-act on the formation of sharp peaks. The complex gave sharp emission peaks in mixed solvent H₂O–DMSO = 99 : 1 and gave smooth emission peaks in DMSO solvent, which shows that the environment and states of the complex influence the sharp shape of emission peaks. The solvent environment, including polarity, dielectric constant (ε₀), refractive index (n), and orientation polarizability (Δf), influence the peaks' shapes. The states of the complex might be the other influential element. The PNS-G0 was well dissolved in DMSO, but it was difficult to dissolve in water. Then the H₂O–DMSO = 99 : 1 mixed solvent system was used to dissolve PNS-G0 to test the fluorescence properties in the simulated biological environment. The PNS-G0 with Zn²⁺ formed the ZnO unit complex in this mixed solvent. The ZnO connected with C=N and naphthalene, giving these HOQDs the narrow emission bands characteristic; then the peaks' shapes were sharp in the mixed H₂O–DMSO = 99 : 1 solvent.

PNS-G0 has primary amine, amide, imine, and hydroxyl groups that can all take part in the complex, so the complex molar ratios with Zn²⁺ were difficult to show clearly. Fig. 13 gives the predicted structures, which were used for quantum chemical calculations. These kinds of HOQDs (Fig. 13: P-02 and P-03) all have inorganic metal ions and organic parts, which gave fluorescence emission, similar to traditional QDs. The HOQDs have organic parts that can give these QDs more organic characters, such as solubility, reactivity, biocompatibility, and more chemically modifiable properties. The PNS-G0 + Zn²⁺ complex has Zn²⁺_imine fluorescence emission groups that can be used as probe labels. The PAMAM cores also can emit fluorescence under some situations, and can package nanoparticles or drugs for delivery. These give (C=N_Zn_O) HOQDs a diverse range of applications.

Fig. 13 The end parts of PNS-G0 and PNS-G0 + Zn²⁺ complex P-01 shows the end parts of PNS-G0; P-02 shows the predicted end parts of PNS-G0 + Zn²⁺ (ligand number 2); P-03 shows the predicted end parts of PNS-G0 + Zn²⁺ (ligand number 4).

The HOQDS have organic parts and inorganic parts. PNS-G0 + Zn²⁺ HOQDs have Zn ions as the inorganic part, and imine conjugated with hydroxyl–naphthalene cycle as the organic part. The QDs have the main characteristic of quantum confinement effect (QCE). HOQDs have quantum confinement effect determined from the Bohr radius compared with the size of in/organic quantum parts. Fig. 13 shows the end parts of PNS-G0 (P-01) and PNS-G0 + Zn²⁺ (P-02 and P-03).

Fig. 14–16 show the molecular orbits of P-01, P-02, and P-03. The calculation used the time-dependent density functional theory (TD_DFT) b3lyp/6-31g methods of the Gaussian 09 software package.³⁴Tables 1–3 list the absorption and emission spectra of P-01, P-02, and P-03 in gas phase vacuum calculated by the TD_DFT b3lyp/6-31g method. S0 → S1 are the absorption states; S1 → S0 are the emission states.

Fig. 14 The molecular orbit clouds and contour figures (HOMO−1, HOMO, LUMO and LUMO+1) of P-01 (end parts of PNS-G0).

Fig. 15 The molecular orbit clouds and contour figures (HOMO−1, HOMO, LUMO and LUMO+1) of P-02 (end parts of PNS-G0 + Zn²⁺, ligand number 2).

Fig. 16 The molecular orbit clouds and contour figures (HOMO−1, HOMO, LUMO and LUMO+1) of P-03 (end parts of PNS-G0 + Zn²⁺, ligand number 4).

Table 1 The absorption and emission spectra of P-01 in gas phase vacuum calculated by TD_DFT b3lyp/6-31g method

Electron transition^a	Energy (eV)	Calculated wavelength (nm)	Main transition configuration	Oscillator strength f
a S0 → S1 are the absorption states of P-01; S1 → S0 is the emission state of P-01.
S0 → S1	2.857	433.93	HOMO → LUMO: −0.62210	0.2138
			HOMO−1 → LUMO: 0.11833
			HOMO−3 → LUMO: 0.31203
S0 → S1	2.940	421.68	HOMO → LUMO:0.30241	0.0794
			HOMO−1 → LUMO: 0.43256
			HOMO−2 → LUMO: 0.33605
			HOMO−3 → LUMO: 0.32663
S0 → S1	3.124	396.87	HOMO → LUMO+2: 0.17661	0.0395
			HOMO−1 → LUMO: −0.28034
			HOMO−2 → LUMO: 0.58927
			HOMO−3 → LUMO: −0.17738
S1 → S0	2.857	433.93	LUMO → HOMO: 0.12852	0.2138

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Table 2 The absorption and emission spectra of P-02 in gas phase vacuum calculated by TD_DFT b3lyp/6-31g method^a

Electron transition^a	Energy (eV)	Calculated wavelength (nm)	Main transition configuration	Oscillator strength f
a S0 → S1 are the absorption states of P-02; S1 → S0 is the emission state of P-02.
S0 → S1	0.220	5624.95	HOMO → LUMO: −1.03080	0.0049
S0 → S1	1.967	630.12	HOMO → LUMO+1: 0.93489	0.0036
			HOMO−1 → LUMO: 0.18387
			HOMO−1 → HOMO: −0.26499
S0 → S1	2.113	586.72	HOMO−1 → HOMO: 0.61623	0.0013
			HOMO−4 → HOMO: −0.12701
			HOMO → LUMO+1: 0.31794
			HOMO−1→LUMO: −0.67219
			HOMO−4 → LUMO: 0.13763
S1 → S0	0.220	5624.95	LUMO → HOMO: 0.26503	0.0049

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Table 3 The absorption and emission spectra of P-03 in gas phase vacuum calculated by TD_DFT b3lyp/6-31g method^a

Electron transition^a	Energy (eV)	Calculated wavelength (nm)	Main transition configuration	Oscillator strength f
a S0 → S1 are the absorption states of P-03; S1 → S0 is the emission state of P-03.
S0 → S1	1.321	938.47	HOMO−2 → LUMO: −0.14309	0.0167
			HOMO → LUMO: 0.69075
S0 → S1	2.297	539.66	HOMO−1 → LUMO: 0.70362	0.0460
S0 → S1	2.318	534.72	HOMO → LUMO+1: 0.70463	0.0030
S1 → S0	1.321	938.47	LUMO → HOMO: −0.13198	0.0167

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These HOQDs have two Bohr radius directions: one is the Bohr radius of Zn ions expended to free space; the other is the imine naphthalene hydroxyl organic conjugated part. The former is the same as the traditional QD Bohr radius. The latter is a π-conjugated organic cycle, which expends the electronic cloud by real atom parts that can be seen in the real atom part Bohr radius. The free space expended Bohr radius and real organic conjugated part Bohr radius form the characteristic HOQDs properties. So the HOQDs might become a new type of QDs to be given more attention.

The P-01 HOMO, LUMO contour figures show that the electronic clouds were distributed at the close side of molecules. The P-02 and P-03 HOMO, LUMO contour figures show the electronic clouds at the Zn²⁺ parts were distributed far away from the Zn atom, and gave a Bohr radius value of about 10 Bohr (B). The C=N_Zn_O six-member cycle form the main part of the HOQDs, which gave size values of about ≈6 Bohr (D). Thus HOQDs obey the D < 2B rule, which shows the quantum confinement effect.

3. Experimental section

PNS-G0 was synthesized from PAMAM-G0 (Fig. 17) and 2-hydroxy-1-naphthaldehyde by Schiff base reaction. PNS-G0 was reacted with the different metal ions (Zn²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Fe³⁺, Fe²⁺, Ni²⁺, Cu²⁺, Co²⁺, Cr³⁺, Ag⁺, H⁺, NH₄⁺, Na⁺, K⁺) in DMSO and water + DMSO (99 : 1) mixed solvent to test the selectivity by UV-vis absorption and fluorescence spectra. PNS-G0 was tested with the different concentrations of Zn²⁺ ions for sensitivity and quantitative detection. The (up-converted) fluorescence was tested for the turn-on effects of probing for Zn²⁺.

Fig. 17 The reaction of PAMAM-G0 molecules.

3.1 The synthesis of polyamides-amine (PAMAM-G0) dendrimers³⁵

Methyl acrylate (MA) and ethylenediamine (EDA) were used as substrates. The Michael-addition of amine groups in EDA to MA under 50 °C in methanol solution affords the dendritic product of −0.5 generation (G) with ester groups terminated. The amidation of the terminal ester groups of the −0.5 G dendrimer from dissolving in methanol solution by excessive EDA under 50 °C affords the 0 G dendrimer with terminal amine groups. Distillation of exceeded EDA under reduced pressure gives the purified 0 G dendrimer. The PAMAM dendrimers are shown as a yellow, ropy liquid.

(PAMAM-G0) IR (KBr) cm⁻¹: 3325 (NH), 2961, 1647 (CO), 1563, 1465, 1307, 995, 687. MS (m/z): (calculate for: 518.6 [M + 2H]²⁺); found: 518.3 [M + 2H]²⁺.

3.2 The synthesis of PNS-G0

The synthesis of 3,3′,3′′,3′′′-(ethane-1,2-diylbis(azanetriyl))tetrakis(N-(2-(((2-hydroxyl naphthalen-1-yl)methylene)amino)ethyl)propan amide) (PNS-G0) is described as follows.

PAMAM(G0), 0.52 g (1 mmol); 2-hydroxy-1-naphthaldehyde, 0.9 g (5 mmol, excessive); and anhydrous sodium sulfate, 0.5 g; were added into 30 mL methanol, refluxed for 2 h and cooled to room temperature. The solution was filtered, and the filtrate was washed by methanol three times, followed by vacuum drying. The result was a yellow brown powder of about 1 g, with a yield of 85%.

IR (KBr) cm⁻¹: 3273 and 3064 (NH), 2936, 2818, 1629 (C=N), 1549, 1495, 1447, 1354, 1210, 1184, 1139, 1104, 998, 835, 739, 505. ¹H NMR (300 MHz, DMSO-d₆, δ): δ 8.95 (4H, m, H–C=N), 8.14 (4H, s, Ar–H), 8.00 (8H, d, J = 6.0 Hz, Ar–H), 7.71 (4H, d, N–H), 7.60 (4H, m, J = 6.0 Hz, Ar–H), 7.38 (4H, t, J = 6.0 Hz, Ar–H), 7.15 (4H, t, J = 6.0 Hz, Ar–H), 6.71 (4H, m, HO–), 3.08–4.01 (24H, m, C–H), 2.17–2.27 (12H, m, C–H). ¹³C NMR (300 MHz, DMSO-d₆, δ): δ 176.9, 171.4, 160.2, 159.5, 137.0, 136.8, 134.1, 128.8, 127.8, 125.2, 125.1, 122.3, 122.1, 118.6, 118.4, 105.8, 50.3, 49.0, 40.3, 40.0, 39.7, 39.5, 39.2, 38.9, 38.6; HRMS (m/z): (calculate for: 1134.3480 [M + H]⁺); found: 1134.348 [M + H]⁺.

3.3 Methods

PNS-G0 solutions were prepared in H₂O–DMSO = (99 : 1) at 1 × 10⁻⁶ mol L⁻¹ or in DMSO at 5 × 10⁻⁵ mol L⁻¹. The different metal ion solutions were added to the PNS-G0 solutions to keep a 1 × 10⁻⁴ mol L⁻¹ ion concentration and form the complex solutions, which were used for quantitative testing. The [Zn²⁺] gradient concentration solutions were prepared and added into PNS-G0 solutions for qualitative detection. For the optical test: the solutions were placed in a quartz color dish, and the fluorescence emission spectra were tested using a linear light source spectrofluorometer. The up-converted fluorescence emission experiments were taken by FluoroMax-4 spectrofluorometer using a xenon excitation source, continuous output, ozone-free lamp (150 W) at room temperature (∼20 °C). The excitation wavelength was set at 800 nm, and test ranges were set from 200 nm to 850 nm, using a 3 nm slit.

4. Conclusions

PNS-G0 can realize up-converted fluorescence turn-on effect for high selectively qualitative and quantitative probing of Zn²⁺. This dendrimer probe is based on the Schiff base imine (C=N) isomerization inhibition mechanism. The use of organic solvent and the nearly two times enhanced selective signal in water–DMSO mixed solvents show the dyes need to be further modified and improved. The C=N_Zn_O fluorescence emission parts were called half-organic quantum dots (HOQDs), which show similar and new properties compared to traditional QDs. The PNS-G0 and its Zn²⁺ complex have potential applications in biological cell imaging, metal ions analytical chemistry, and up-converted optical physics areas. Furthermore, the HOQDs can be used to explore a wide variety of new applications.

Acknowledgements

The National Natural Science Foundation of China (no. 61178057) and the Scientific Research Foundation of Graduate School of Southeast University (no. YBPY1209) were greatly appreciated for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR spectra, HRMS, and IR of PNS-G0 listed in Fig. S1–S4. MS and IR of PAMAM-G0 listed in Fig. S5, S6. See DOI: 10.1039/c4ra01758g

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