Dual-sensing porphyrin-containing copolymer nanosensor as full-spectrum colorimeter and ultra-sensitive thermometer

Abstract

A porphyrin-containing copolymer has dual-sensing in response to metal ions and temperature as a novel nanosensor. Triggered by ions, the sensor exhibits full-color tunable behavior as a cationic detector and colorimeter. Responding to temperature, the sensor displays an “isothermal” thermochromic point as an ultra-sensitive thermometer.

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The design of full-color-tunable optical sensors is of increasing demand due to their ultra-sensitive full-spectrum chromatism.¹ Such smart sensors have a wide range of applications in organic analysis,² DNA sequence detection,³ and as thermometers,⁴ display devices,⁵ and bar codes.⁶ At present, a universal approach to obtain the desirable multicolors is the use of nanocrystals such as gold (Au), silver (Ag), and semiconductor quantum dots.⁷ However, the color of these nanocrystals is not really “tunable”, and they are generally synthesized under harsh conditions and usually suffer from cytotoxicity and leakage into the biological system. To address this issue, stimuli-responsive chromatic materials have been proposed as an alternative route to fulfill the color modulation. Although great efforts have been recently devoted to the simple- or multiple-responsive chromatic switches,⁸ the study of soluble nanosensors that have distinct color transitions and broad color-tuning range (>three kinds) is still a hot area of study.⁹

In a previous study, we discovered a soluble logical copolymer in which the single fluorophore at the junction of responsive blocks responds to various aggregated states of the outer or inner blocks with temperature changes.¹⁰ In this context, based on the principle of specific chromophore location and polymer phase induction, we first report on a full-color polymer optical nanosensor, possessing four prominent advantages: (i) metal-ion triggering of a full-spectrum-tunable sensing pattern which can enable use as an ion detector and as a light colorimeter at ambient temperature; (ii) each ion-containing polymer sensor possesses a distinct thermochromic temperature so enabling a broad range as an ultra-sensitive thermometer; (iii) dispersive polymeric nanoparticles with high water-solubility acting as sensing unit, maintaining a discrete signal transport; (iv) full-color modulation so that the sensor is capable of sensing multi-channel signals, and (iv) low cytotoxicity for application in biosystems.

To these purposes, we have designed and synthesized a specific ABC triblock copolymer in which thermomorphic block poly(N-isopropylacrylamide) (PNIPA) and hydrophilic poly(methacrylic acid) (PMAA) mediates the shortest poly(2-hydroxylethyl methacrylate) (PHEMA) bearing the pendants of tetra(4-carboxylatophenyl)porphyrins (TCPP) (PNIPA-b-PHEMA-b-PMAA, 1, Scheme 1) by sequential reversible addition fragmentation chain transfer polymerization (ESI†).^10,11 Surface tension measurement gave a critical micelle concentration (∼1.20 g L⁻¹) in aqueous media and the average hydrodynamic radius 〈R_h〉 is 29.4 nm by dynamic light scattering monitoring (DLS), indicating a good solubility and the formation of dispersively spherical micelles (Fig. S1 and S2, ESI†).

Scheme 1 Schematic illustration of ABC triblock copolymer structure and its phase transition behavior inducing the TCPP species oriented aggregation upon temperature-stimuli.

Thermochromic properties of 1 as shown in Fig. 1 (left) are seen at various temperatures. Below the low critical soluble temperature (LCST) of PNIPA (∼32 °C), a transparent red–brown solution with a diagnostic TCPP Soret band (λ_max = 434 nm) was observed. Upon heating above 35 °C, interestingly, a clear blue-shift from 434 to 406 nm is seen and is accompanied by a visible turbid transition (transmission: 100% to 20%) by UV-vis monitoring (Fig. S3 and S4, ESI†), implying an aggregated state transition around TCPP species. One plausible reason of the thermochromic phenomenon is the micro-environment polarity variation and the oriented aggregation of TCPP species (Scheme 1). DLS revealed the 〈R_h〉 of the nanosensor changed from 29.4 to 16.3 nm upon the temperature increase (Fig. S5, ESI†), furthermore, the solution polarity decreases dramatically with reduction of the micellar radius, supporting the hypothesis of environment polar induction.

Fig. 1 Simplex-stimuli-sensing thermochromic sensor in the absence of metal ions (left), and dual-sensing optical sensors (full-spectrum colorimeter and ultra-sensitive thermometer) upon introduction of different metal ions (right).

To ensure the thermochromism of the copolymer. we then investigated as to how to further enlarge the stimuli-responsive full-color range. Below 32 °C, addition of different metal ions to the solution of 1, leads to an unprecedented full spectral colour range (Fig. 1, top) with the Soret π → π* absorption of the TCPP species showing large variation from 418 to 512 nm upon metal ion coordination with TCPPs (Fig. 2(a)). Even though the metalloporphyrin has a different color than the porphyrin, a full-spectrum color-tuning sensor by cation-stimulus TCPP-containing block copolymer is shown for the first time.¹² Most surprisingly, on heating these multicolor nanosensors, they display discrete thermochromic characteristics in the temperature range of 35–61 °C with the phase transition point dependent on the metal ion (Fig. 1 and Fig. 2(b)). As compared to the reported thermochromic materials,^4,8c the color transition interval (Δλ_max >37 nm) is very large and the visual color change is very distinct. It is notable that the thermochromism speed is typically less than ∼30 s, and the temperature range of the color transition is smaller than 0.4 °C. Another ultra-sensitive feature of the sensors is an abrupt turbidity switch at the phase transition point (transmission: 100% → 0%, Fig. 2(c)). Cyclic thermal curves show that the thermochromism is fully reversible (Fig. 2(d)).

Fig. 2 (a) UV-vis absorption of TCPP-containing triblock copolymer 1 exhibiting full-spectrum color changes triggered by different metal ions at room temperature (32 °C). (b) A broad range of ultra-sensitive thermochromic properties of copolymer 1 with different metal ion triggers displaying an “isothermal” responsivity (35–61 °C). (c) Turbidity of TCPP-containing copolymer 1 with different metal ions at the thermal phase transition. (d) A typical cyclic thermal curve displaying recycled thermochromic behavior switched from 430 nm (32 °C) to 481 nm (43 °C) (copolymer 1 with Cu²⁺).

Our next aim focuses on proposing a reasonable mechanism of the full-color-tunable polymeric optical sensors. Two key problems must be addressed: (i) why is the thermal-induced color transition of the metalloporphyrin-containing block copolymer large, and (ii) why does the phase transition temperature exceed the LCST of PNIPA (∼32 °C),¹³ showing an “isothermal” responsivity from 35 °C to 61 °C by different cationic triggers (Fig. 2(b)). We infer that an ion-induced LCST increase is the significant reason. As reported, several ions in solution can act as kosmotropes (water “structural maker”) that induce the formation of H-bonds between PNIPA and H₂O mediated by hydrated ion centers.¹⁴ In the absence of metal ions, a normal process is that the amido groups of PNIPA interact with H₂O via NH⋯OH₂ intermolecular H-bonds at low temperature whereas above the LCST, PNIPA chains strongly collapse to exclude internal H₂O and recreate H-bonds among amido groups of PNIPA each other via NH⋯NH interactions (Fig. 3(a)). In contrast, upon introduction of metal ions, besides partial cation coordination with TCPPs, most of them form hydrated ion-polymer complexes (M(H₂O)_mⁿ⁺⋯NH⋯M(H₂O)_mⁿ⁺), which interact with many more amido groups of PNIPA together through the synergism of ion–dipole forces between metal ions and amido groups and H-bonds between PNIPA and H₂O (Fig. 3(b)). These ion–polymer complexes could be regarded as adhesive centers to connect PNIPA groups compactly. Disrupting the ion–polymer complexes requires energy and so results in a LCST increase. Moreover, different ions will show different association strengths with the polymer, which causes an “isothermal” responsivity from 35 to 61 °C. Accompanied by the stronger PNIPA phase transition, the micro-environment polarity around TCPP species will change significantly at the LSCT, which leads to a significant color change.

Fig. 3 (a) Schematic illustration of normal PNIPA chains below and above the LCST in the absence of metal ions. (b) Modified situation of PNIPA chains with hydrated ion–polymer complexes below and above the new LCST” in the presence of metal ions.

In comparison with the multicolor nanocrystals, our polymeric optical nanosensor possess real color-tunability by metal ion triggers and special thermochromism upon temperature stimulus. If we use these copolymer solutions with various metal ions (Fe³⁺, Fe²⁺, Cu²⁺, Mn²⁺, Mg²⁺, Co²⁺, Cd²⁺, Ni²⁺, and Zn²⁺) to establish a nanosensor array, it is capable of simultaneously conveying nine-channel color signals or light information imputs. Compared to the reported responsive materials, this nanosensor has a longer-range of thermochromic behavior (35–61 °C) that significantly expands the temperature region as a smart thermometer array.

In summary, the synthesis and evaluation of responsive copolymers bearing TCPP species demonstrates that it possesses conceivable thermochromism in the absence of metal ion stimuli. Upon modification by metal ions the available color range is hugely extended and upon heating the solutions, color transitions at different thermochromic points are observed. It is anticipated that this polymeric optical nanosensor can open new insights on full-spectrum colorimetric and ultra-sensitive thermometric arrays.

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 20836004, No. 20974058) and the National Basic Research Program (2009CB930602).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and characterization details. See DOI: 10.1039/b926882k

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