Luminescence “Turn-On” Detection of Gossypol Using Ln³⁺-Based Metal–Organic Frameworks and Ln³⁺ Salts

Abstract

Gossypol (Gsp), a natural toxin concentrated in cottonseeds, poses great risks to the safe consumption of cottonseed products, which are used extensively throughout the food industry. In this work, we report the first luminescence “turn-on” sensors for Gsp using near-infrared emitting lanthanide (Ln3+) materials, including Ln3+ MOFs and Ln3+ salts. We first demonstrate that the Yb3+ photoluminescence of a Yb3+ MOF, Yb-NH2-TPDC, can be employed to selectively detect Gsp with a limit of detection of 25 μg/mL via a “turn-on” response from a completely nonemissive state in the absence of Gsp. The recyclability and stability of Yb-NH2-TPDC in the presence of Gsp was demonstrated by fluorescence spectroscopy and PXRD analysis, respectively. A variety of background substances present in practical samples that would require Gsp sensing, such as refined cottonseed oil, palmitic acid, linoleic acid, and α-tocopherol, did not interfere with the Yb3+ photoluminescence signal. We further identified that the “turn-on” of Yb-NH2-TPDC photoluminescence was due to the “antenna effect” of Gsp, as evidenced by spectroscopic studies and supported by computational analysis. This is the first report that Gsp can effectively sensitize Yb3+ photoluminescence. Leveraging this sensing mechanism, we demonstrate facile, highly sensitive, fast-response detection of Gsp using YbCl3·6H2O and NdCl3·6H2O solutions. Overall, we show for the first time that Ln3+-based materials are promising luminescent sensors for Gsp detection. We envision that the reported sensing approach will be applicable to the detection of a wide variety of aromatic molecules using Ln3+ compounds including MOFs, complexes, and salts.

Ligand engineering to achieve enhanced ratiometric oxygen sensing in a silver cluster-based metal-organic framework

“…The significant decrease in the ligand-based Fl lifetimes of Ag 12 bpy-NH 2 from 3.59 to 0.37 ns, which is accompanied by a considerable increase in the Ph lifetime from 0.2 μs to 3.12 ms (Table 1), suggested the occurrence of an efficient ISC process from the lowest singlet to the lowest triplet excited states (vide infra). The 15,000-fold enhancement in the Ph lifetime is an unprecedented phenomenon among luminescent MOFs [19][20][21][22][23][24][25][26][27][28][33][34][35][36][37][38] and metal clusters [39][40][41][42][43] , even in metal-containing phosphors 12,13,[44][45][46] , which means that the triplet excited state(s) of Ag 12 bpy-NH 2 is capable of responding to oxygen molecules with a sensitivity of one-15,000th of the original sensitivity of unsubstituted Ag 12 bpy if one merely considers the lifetime factor. Of course, this also presents a challenge for obtaining precise measurements in the range of hypoxia.…”

“…6). Ag 12 bpy-NH 2 , with an LOD as low as 0.11 ppm, is applicable for nearly anoxic systems with oxygen levels in the range of 0.1-24 ppm and has sensitivities that are orders of magnitude above those of conventional sensing MOFs [19][20][21][22][23][24][25][26][27][28][33][34][35][36][37][38] . Ag 12 bpy-NH 2 opens the door to completely new applications for monitoring oxygen in previously inaccessible concentration regions and is likely to become invaluable in diverse fields of science and technology.…”

“…(~8.5 × 10 6 times) of the Ph and Fl bands, combined with the highly oxygen-permeable framework structure, enable Ag 12 bpy-NH 2 to function as a single-component Fl-Ph ratiometric sensor for molecular oxygen in which ultrafast responses towards trace oxygen gas are achieved in 0.3 s, and the limit of detection (LOD) is as low as 0.1 ppm; this response can be visualized by variations in the color, which is dependent on the O 2 concentration below 20 ppm. To the best of our knowledge, Ag 12 bpy-NH 2 is the first report of a Fl-Ph dual-emissive MOF that has a record-breaking LOD and response speed [8][9][10][19][20][21][22][23][24][25] . By the further introduction of bulk −CH 3 groups on the partial bpy linkers, which interferes with the oxygen collision dynamics, we then prepared another isostructural mixed-linker Ag 12 bpy-NH 2 /CH 3 , which extended the ratiometric sensing range.…”

“…Metal-organic frameworks (MOFs) feature flexible secondarybuilding units (SBUs) and infinite tunable organic linkers, as well as abundant and distinctive pores or channels, and hence have shown great potential for separation and sensing [8][9][10][11][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] . Traditionally, dual emission in MOFs is mostly achieved with lanthanide ion or luminescent linker on a host MOF framework as one emissive center and a post-introduced luminescent species including lanthanide ion, dye or carbon dot as another emissive center, which make it difficult to realize ultrasensitive and ratiometric detection towards oxygen gas 11 .…”

“…Silver-chalcogenolate-cluster-based metal-organic frameworks (SCC-MOFs) [33][34][35][36][37][38] , in which each SBU consists of tens of silver and chalcogen atoms, have become a new member of the MOF family. SCC-MOFs combine the advantages of silver clusters [39][40][41][42][43] and MOFs [8][9][10][11][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] ; SCC-MOFs have shown remarkably improved luminescent properties relative to discrete silver clusters and unique excellent luminescent responses for sensing gases and volatile organic compounds (VOCs) [33][34][35][36][37] . We previously reported a SCC-MOF, that is, Ag 12 bpy 33 , ([Ag 12 (S t Bu) 8 (CF 3 COO) 4 (bpy) 4 ] n , bpy = 4,4′-bipyridine), whose single green Ph is sensitive to oxygen (Fig.…”