Efficient Fluorescence detection of Fe&lt;sup&gt;3+&lt;/sup&gt;, Cd&lt;sup&gt;2+&lt;/sup&gt;, and CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; based on an azine-Functionalized Zn-Metal-Organic Framework