Four new fluorescent sensors (1–4) based on the 4-amino-1,8-naphthalimide fluorophores (Naps) have been synthesized based on the classical fluorophorespacer-receptor model. These four compounds all gave rise to emission bands centred at ca. 535 nm, which were found to be highly pH dependent, the emission being ‘switched on’ in acidic media, while being quenched due to PET from the amino moieties to the excited state of the Nap at more alkaline pH. The luminescent pH dependence for these probes was found to be highly dependent on the substitution on the imide site, as well as the polyamine chain attached to the position 4-amino moiety. In the case of sensor 2 the presence of the 4-amino-aniline dominated the pH dependent quenching. Nevertheless, at higher pH, PET quenching was also found to occur from the polyamine site. Hence, 2 is better described as a receptor₁-spacer₁-fluorophore-spacer₂-receptor₂ system, where the dominant PET process is due to (normally less favourable) ‘directional’ PET quenching from the 4-amino-aniline unit to the Nap site. Similar trends and pH fluorescence dependences were also seen for 3 and 4. These compounds were also tested for their imaging potential and toxicity against HeLa cells (using DRAQ5 as nuclear stain which does now show pH dependent changes in acidic and neutral pH) and the results demonstrated that these compounds have reduced cellular viability at moderately high concentrations (with IC₅₀ values between ca. 8–30 µmol·L⁻¹), but were found to be suitable for intracellular pH determination at 1 µmol ·L⁻¹concentrations, where no real toxicity was observed. This allowed us to employ these as lysosomal probes at sub-toxic concentrations, where the Nap based emission was found to be pH depended, mirroring that seen in aqueous solution for 1–4, with the main fluorescence changes occurring within acidic to neutral pH.

中文翻译：

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基于4-氨基-1,8-萘二甲酰亚胺的荧光光致电子转移（PET）pH传感器作为脂质体细胞显像剂：取代基图案对PET定向淬灭的影响

基于经典的荧光团-间隔子-受体模型，已经合成了基于4-氨基-1,8-萘二甲酰亚胺荧光团（Naps）的四个新的荧光传感器（1-4）。这四个化合物都产生了以约中心的发射带。535 nm是高度依赖pH的，在酸性介质中发射被“打开”，同时由于PET从氨基基团到更碱性pH的Nap的激发态而被猝灭。发现这些探针的发光pH依赖性在很大程度上取决于酰亚胺位点上的取代以及与位置4-氨基部分相连的多胺链。在传感器2的情况下4-氨基苯胺的存在决定了pH依赖性猝灭。然而，在较高的pH值下，也发现在多胺位点发生了PET猝灭。因此，2被更好地描述为受体₁间隔子₁ -fluorophore间隔物₂受体₂的系统，其中，占主导地位的PET工艺是由于（通常不太有利）“定向” PET从4-氨基苯胺单元以淬火在午睡的网站。3和4也观察到类似的趋势和pH荧光依赖性。还测试了这些化合物对HeLa细胞的成像潜能和毒性（使用DRAQ5作为核染色剂，现在确实显示了酸性和中性pH值的pH依赖变化），结果表明，这些化合物在中等高浓度下具有降低的细胞生存力（ IC ₅₀值大约在8–30 µmol·L ^-1）之间，但发现适合在1 µmol·L ^-1浓度下进行细胞内pH测定，但未观察到真正的毒性。这使我们能够以亚毒性浓度将其用作溶酶体探针，发现基于Nap的发射依赖于pH值，反映了水溶液中1-4的情况。，主要的荧光变化发生在酸性至中性pH范围内。